Forty minutes down the interstate, somebody flashes their lights at you. You find somewhere to pull in, walk back along the side of the rig, and there it is: the pass-through bay standing open, the door swinging on its stops, and a shallower pile of things inside than there was this morning.
Nothing about that morning felt rushed. You closed the bay. You are fairly sure you closed the bay. The latch on that door has always needed a shove, and the difference between shoved and shut is about three millimetres and no sound at all.
This is the kind of problem a five dollar magnet solves completely, and the kind that no amount of being careful solves at all. It is worth being precise about which problems those are, because the sensor that catches the bay door is the same sensor people buy expecting a burglar alarm, and it is not one.
What we read
Door and motion sensors reach RoamVitals through BTHome v2, which is an open broadcast format rather than a brand. A sensor that speaks it puts its state on the air every time that state changes, and anything listening can read it. No account, no cloud, no pairing, no app in the middle.
That gets us four things worth having:
- Open or closed. Binary and physical. The magnet is beside the reed switch or it is not.
- Motion. A passive infrared sensor firing, which is a narrower claim than it sounds. More on that below.
- Light level. Sent by most of these sensors and routinely ignored. It is the most useful of the four in a storage bay.
- Battery. So the sensor can tell you it is about to stop being a sensor.
Verified against the specification, not against every device. We have implemented BTHome v2 as documented and we read it correctly. We have not personally opened every sensor that claims to speak it. If it broadcasts BTHome v2, RoamVitals reads it; whether a particular unit is well built is a separate question and not one we can answer for you.
This is not a security system, and the reason matters
A sensor broadcasts to the hub inside your rig. The hub tells you. That second step needs a working internet connection at the RV, and an RV parked in a storage lot in February has no internet at all.
So the honest description of what you get is this. On shore power at a campground with wifi, or tethered to a phone, a door opening reaches you in seconds. In a storage lot with nothing, the hub records every event with a timestamp and hands you the whole history the moment it is back on a network, which in practice is when you next visit. Cellular would close that gap and it is planned rather than available.
That still has real value. Knowing on Saturday that the bay was opened on Tuesday is worth a great deal more than never knowing. It is not worth anything at all as a deterrent, and a sensor that reports a theft four days later has not prevented a theft. If what you want is somebody notified while it is happening, you want a monitored alarm with its own cellular radio, and you should buy one instead of this.
The four placements that actually earn their keep
1. The storage bay you have to shove
Every rig has one. It is the single best use of a door sensor and almost nobody buys one for it, because the failure is embarrassing rather than dramatic and people assume they will just be more careful.
The value is entirely in the timing. A bay checked at the moment you pull the levelling jacks up is a two second fix. The same bay checked forty minutes later is a police report.
2. The fridge
An RV fridge door is held by a friction catch that was designed decades ago and is often the original. It comes open on a corner, sits ajar for six hours of driving, and everything inside is warm and quietly unsafe by the time you stop.
A door sensor catches the cause. A temperature sensor on the middle shelf catches the consequence, which is the thing that actually decides whether dinner is a problem. Both together is one of the few genuinely complete pairings in RV monitoring: you learn that the door opened, and separately whether it mattered.
3. Confirming somebody came
If a friend is checking on the rig over the winter, a door sensor tells you they did, and when. This sounds like distrust and is not: it is the difference between believing the rig has been looked at and knowing it, and it means the one week nobody could make it is a week you know about rather than assume away.
4. The dog, and whether the door held
If you leave a dog in the rig, the entry door staying shut is not a small detail. A screen door latch that pops is how a dog ends up in a car park. This pairs with the temperature you are already watching, and it answers a question temperature cannot: not is it still cool in there, but is the dog still in there.
Motion sensors lie more than door sensors do
Not a fault, a limitation of what a passive infrared sensor is. It watches for a moving warm shape against a cooler background. Inside a parked RV, several ordinary things look exactly like that:
- Sun through a window. A patch of sunlight crossing the floor through the afternoon is a warm shape that moves. This is the most common false trigger by a wide margin.
- A curtain near a roof vent. A vent left cracked moves fabric, and fabric warmed by sun is warm and moving.
- Your pets. A dog is not a false trigger at all. It is a correct detection of a thing you already knew about, arriving as an alert every twenty minutes.
- The fridge cycling. Less common, but a vent blowing warm air across the sensor’s field will do it.
Two things make motion useful anyway. Point it at a doorway rather than across a room, so it is watching the one metre something has to pass through instead of everywhere sun can land. And read the light level alongside it, because a motion event with a light level that jumped at the same moment is a door opening, while a motion event in unchanged darkness is much more likely to be the curtain.
If you only buy one, buy the door sensor. Open is a fact. Motion is an inference.
What to buy
The Shelly BLU range is the straightforward answer: it broadcasts BTHome v2 out of the box, it is sold as a normal consumer product rather than a kit, and it needs nothing flashed or configured to be readable. Three are worth knowing about. The BLU Door/Window is the one to buy first, for the bay and the fridge. The BLU Motion covers a doorway and reports light level alongside movement. The BLU H&T is there if you would rather have temperature and humidity from the same family than manage two.
Anything else that speaks BTHome v2 works identically, including a pvvx-flashed sensor or a home-built ESPHome node. There is no allowlist on our side. We read the format.
Encrypted BTHome needs its key. Some devices ship with encryption enabled, and an encrypted broadcast is unreadable noise until its bind key is saved against the sensor in the hub. If a sensor appears and never produces a reading, this is almost always why.
Two practical notes
Mind the gap on the magnet. Bay doors flex, and the alignment that works in the driveway can drift enough on a rough road to report open when it is not. Mount the two halves closer than the maximum spec, not at it.
Coin cells hate the cold. A CR2032 loses a lot of usable capacity below freezing, and an exterior bay is colder than the rig. A sensor that has been silent since the first hard frost has almost certainly not failed; it is just cold, and the battery percentage it last reported will have looked fine.
The short version
Buy a door sensor for the bay that needs a shove, and a second for the fridge. Add temperature inside the fridge, because the door opening and the food spoiling are two different facts and only one of them is the one you care about. Treat motion as a supporting signal rather than a headline, and point it at a doorway.
And do not buy any of it as a burglar alarm. It tells you what happened to your rig. Without a connection at the RV it does not tell you quickly, and being told on Saturday about Tuesday is a genuinely useful thing that is not the same as being protected.
