Most people buy their second sensor for a reason and their first one at random. The second one is always the interesting purchase, because something went wrong and now there is a specific question worth answering. This is a list of those questions, in the order they are actually worth spending money on.
It is not a list of everything that can be measured. Plenty can be measured and tells you nothing you will act on, and a dashboard of numbers nobody reads is worse than three numbers somebody does.
The test for whether something is worth monitoring
Two questions, and it has to pass both.
Would you find out too late otherwise? A dead battery announces itself when you try to use it. A slowly dying battery does not. The second is worth a sensor; the first mostly is not.
Could you do anything about it? Knowing the fridge is at 12 degrees is only useful if you can get to it, call somebody who can, or write off the contents on purpose rather than by surprise. If the honest answer is that you would read the alert and do nothing, that is a number for a dashboard, not an alert.
Tier one: buy these first
Temperature, in two or three places
The cheapest sensor and the one that answers the most questions. Interior temperature is the one people buy; the wet bay is the one that saves a winter, and the fridge is the one that saves a weekend.
Buy three cheap ones rather than one good one. Position beats precision here by a wide margin, and a sensor half a degree out in the right cupboard is worth more than a laboratory-grade one in the wrong room. Which ones we read.
Humidity, which comes free with temperature
Almost every temperature sensor worth buying reports humidity too, so this costs nothing extra and is the early warning for mould in a closed-up rig. Sustained high humidity in storage is a problem you can still fix. The stain on the ceiling three months later is not.
Shore power
Pedestal power at a campground is not reliable, and the damage is not usually a dramatic surge. It is low voltage, which makes motors work harder and quietly kills the air conditioner over a season.
A protector that disconnects is doing the important half of the job whether or not it reports anything. Reading it as well tells you the pedestal is marginal before the compressor tells you.
Tier two: buy these once you know your rig
Battery state of charge, properly measured
Voltage is a bad proxy and a lithium battery makes it worse: the voltage sits nearly flat across most of the usable range and then falls off a cliff. A shunt counts amps in and amps out, which is the only method that survives contact with LiFePO4.
Tier two rather than tier one only because plenty of people are on shore power most of the time. If you boondock at all, move it to tier one.
Tank levels
Factory tank senders are famously wrong, usually because the black tank probes are covered in the thing they are trying to measure. External ultrasonic sensors and replacement sender boards both dodge that. Propane is the one worth doing first, because running out of propane in February is a different class of problem from running out of fresh water.
Tyre pressure and temperature
Pressure is what people buy it for. Temperature is what actually warns you: a bearing about to fail or a tyre coming apart heats up before it goes, and that is minutes of notice on a failure that otherwise arrives all at once at highway speed.
Leak detection under the wet bay
Cheap, and the failure it catches is the most expensive one an RV has. Water writes off more RVs than fire does, slowly, from the inside. A puck under the water heater and one beside the pump covers most of where plumbing actually fails.
Tier three: specific problems, specific answers
Doors and storage bays
The most commonly overlooked sensor on this list, and one of the cheapest. The bay door that needs a shove, the fridge that comes open on a corner, confirmation that the friend checking on the rig actually came. Not a burglar alarm, and worth reading why before you buy one expecting to be.
Motion
Useful pointed at a doorway, unreliable pointed across a room. Sun moving across a floor looks exactly like a person to a passive infrared sensor. Supporting evidence rather than a headline.
Air quality and CO2
A closed rig with people asleep in it reaches CO2 levels that make you feel like you slept badly, and the same condition puts the water on the walls that gets mistaken for a leak. CO2 is not carbon monoxide and the two get conflated constantly, which is worth being clear about before buying either.
Air conditioner start current
Narrow, and genuinely useful if you run a generator or a 30 amp pedestal. A soft starter reports its inrush and fault counts, which turns “the air conditioner sometimes will not start” into a number that changes over time.
What we do not read, stated plainly
A list like this is only useful if it also says where it stops.
- Smoke, carbon monoxide and LP alarms. The certified alarms in an RV have no radio in them at all, deliberately. We cannot read them and neither can anything else, and no monitoring product should ever be treated as a substitute for one.
- Roof and seal leaks. Water that gets in at a vent runs down inside a wall and surfaces somewhere unrelated. Nothing electronic catches this reliably. Twice a year on the roof with your eyes is the actual answer.
- Levelling. Levelling devices are not tank sensors and we do not decode them.
- Solar controller output on some Victron devices. The advertisement is recognised, the solar payload is not decoded yet. Named rather than glossed over.
- Anything that needs a phone in range. A sensor that only talks to its own app over a live connection is not a sensor we can read while you are three states away.
None of this is life safety. RoamVitals is a monitoring tool. It must never replace certified life-safety equipment, and a working smoke, CO and LP alarm is not optional because you have sensors.
The order, if you want it as a list
Three temperature sensors. Shore power. Then whichever of battery, tanks, tyres or leaks matches how you actually use the rig, which you will know by then. Doors when you notice the bay that needs a shove, and you will notice.
Every one of these reads a sensor you buy from somebody else and own outright. The full list of what we decode says which specific devices, and what each one actually reports rather than what the box claims.
