Fresh going down, gray coming up

Third morning off grid, somebody is in the shower, and the water stops. Or you come back from a walk to find an inch of gray water standing in the shower pan, which means it has been over the lip of the tank for a while and the floor has been getting it. Both are the same failure: a number nobody was watching crossed a line nobody had set.

Tank levels are the least glamorous readings in an RV and among the most useful, because they change slowly and predictably. A tank that is going to be a problem on Thursday is already telling you on Tuesday. And the two directions together — fresh going down, gray coming up — say things that neither number says alone, which is why they belong in one article.

Reading the tanks

a. The SeeLevel board you may already own

Garnet’s SeeLevel II is the factory tank monitor in a great many coaches, and the Bluetooth versions broadcast what the panel shows. If yours is one of them, this costs you nothing at all: the 709-BTP7 sends fresh, gray, black and propane together every 120 seconds. The older 709-BTP3 carries the three water tanks but no propane at all, and it cycles one sender per broadcast, so its readings arrive staggered by a few polls.

Example thresholds, which are the app’s own defaults: fresh below 20 per cent warns and below 8 per cent is critical; gray and black warn at 75 per cent and go critical at 90. Set the fresh warning at the level where a fill trip is still convenient rather than at the level where it is urgent — that is the entire value of the alert.

b. A Mopeka under the tank, if you have no board

The Mopeka magnets to the underside of a tank and measures the liquid ultrasonically, with no plumbing and no drilling, which makes it the only retrofit here that does not involve dropping a sender strip on the outside of a tank you cannot reach. It reads every 300 seconds — slow, and perfectly adequate for something that fills over days.

Two things to know before you order. The water version is a different part from the propane one, and they look alike; make sure the listing says H2O. And on water it reports a raw level in millimetres rather than a percentage, because water needs its own sound-speed curve and that curve has not shipped. So you calibrate it yourself: watch one full fill and one drain, write down the number at full and the number at empty, and set the alert against those. Alert on reading quality too, which the sensor broadcasts honestly rather than guessing — a poor echo off a painted or ribbed tank base is the sensor telling you not to believe the level.

c. A puck on the bathroom floor

A leak puck beside the shower or at the base of the toilet is the backstop for the day a sender lies to you. Put it on the floor the water would spill onto, not in the shower pan, where every shower would fire it. It is late by definition — the tank has already overflowed — but it is the one alert on this page that does not depend on a tank reading being correct. There is more on placement in the leak detection guide.

d. Pump current, as half of an answer

A demand pump on an empty tank never stops, and a pump run dry does not last. If you already have a shunt, current staying above your empty-rig baseline across several consecutive polls is that signature. On its own it means only that something is drawing; a whole-house shunt sees the furnace and the fridge too. Paired with a tank reading it is unambiguous, which is the next section.

One more worth naming: Lippert OneControl is recognized, and its frames are visibly channelised sensor slots that look like tanks. Which channel is which tank has not been mapped, so nothing decodes and there is nothing to alert on. If you have a OneControl coach and would compare our channel numbers against what the Lippert app shows, that is the whole of what it needs.

The pairs that say more than either number alone

The shape the whole cookbook is built on: one reading says something happened, two say what.

• Fresh falling, gray not rising, nobody in the rig — water left the fresh tank and did not go down a drain. That is a leak, and the fresh tank found it first.
• Fresh falling, gray rising by the same amount — somebody is using water. Normal if you are there. If you are not, a tap is running into a sink and the gray tank fills before the fresh one empties.
• Fresh empty and the pump will not stop — it is running dry and every minute costs it. Switch the pump off first; the tank can wait.
• Gray above 90 with the valve open on hookups, wet bay below freezing — the sewer hose is frozen and nothing is draining. Stop showers until it thaws, and expect the hose to have split.
• Gray above 90 with the valve open, bay warm — a kinked hose, a valve that did not really open, or the site’s sewer is blocked. Go and look at the hose before anything else.
• Black unchanged for days while the rig is occupied and fresh keeps dropping — the sender is misreading, not the tank. Dump on a schedule until it is fixed.

What none of this tells you

Nothing we read knows whether a dump valve is open or closed, whether the sewer hose is connected, or whether a sender has fouled. A gray tank at 90 per cent with the valve open on full hookups is a completely different problem from one with the valve shut, and only you know which you are looking at.

There is also no alert for a reading that has not moved in a week. A stuck sender and a correct one look identical to the app, and that is the failure mode to keep in the back of your mind — it is why the last item in the list above is something you notice on the history chart rather than something that pages you.

And the alert is on a level, not a rate. “You have a day of water left” is a judgement you make by glancing at the slope on the chart; nothing here calculates it for you. Senders also read through the tank wall, so a rib, a sticker or a patch of foam behind the strip makes a tank jump or stick, and a rig parked on a slope reads one end of a tank. Level it before you believe the last ten per cent.

What to do about it

If the coach has a Bluetooth SeeLevel board, you are done — turn the alerts on, check the labels once, and it has cost you nothing. If it does not, a Mopeka on the fresh tank is the retrofit that needs no plumbing, with the raw-level calibration as the price of that.

Whatever you fit, dump on a schedule rather than on the alert. The alert is there for the week the schedule is wrong — the guests, the long weekend, the sender that quietly stopped moving — and a tank routine that depends on a reading being right is a routine with a single point of failure in it. That is also the argument for the floor puck: not as a plan, as the thing that catches the day the plan was wrong.

Where to get one

Most readers of this page already own the answer and only need to switch it on. For everyone else, in order. One warning worth repeating: the Bluetooth SeeLevel listing we could find does not state its board number, so if you are buying rather than reading one you already have, confirm the model before you order — it is the difference between every tank at once and one at a time. The compatibility database has both boards side by side.