A customer finds a wet cabinet floor and says, "I guess we have to tear the wall out." Almost never — at least not first. Tearing in blind is how a $20 fitting turns into a wall, a floor, and a lost weekend.
I find leaks in the order the water moves: prove which system is leaking, narrow it to one run, then pinpoint the fitting. Only then do you open anything.
Step 1: Let the pump tell you if it's a fresh-water leak
Fill the fresh tank, close every faucet, and turn the pump on. Bleed air from each faucet until water runs steady, close them, and let the pump shut off. Then listen for five to ten minutes — nobody flushes, nobody washes hands.
In a tight system, the pump stays quiet. If it cycles in short bursts with no water being used, pressure is bleeding off in the fresh lines — a fitting, the water heater connections, the pump check valve, or a drip you haven't found. That free test tells you you're hunting fresh water, not rain and not a drain. If it never cycles but you still find water, stop chasing pressure lines: it's probably a drain, a toilet seal, or an outside leak.
Step 2: Split the system in three
Every water problem lives in one of three places: pressurized fresh water, drains, or city water. Test on tank and pump first, then switch to city water through a regulator. A leak that only appears on city pressure — or a fresh tank that fills by itself — points at the city inlet or a check valve.
For drains, shut the pump off and run one fixture at a time. Fill the bathroom sink, pull the plug, and watch underneath. Then the kitchen sink. Then the shower. One fixture, one test, one answer.
Step 3: Dry everything, then wrap it in paper towels
Dry every fitting you can reach — under sinks, behind the toilet, at the water heater, pump, and wet bay — until it's bone dry. Wrap a dry paper towel around each one and tape it in place.
Pressurize the system or run the fixture, then check in fifteen minutes and again in a few hours. A damp towel is your leak, even when the drip is too slow to see. Water travels along PEX and drips off the low point, so the puddle is often not under the leak — the wet towel on the high fitting is.
Step 4: Map it with a moisture meter before you cut
Water soaks farther than it stains. I map it with a pinless meter like the Klein Tools ET140 Pinless Moisture Meter, which reads up to 3/4 inch deep without pin holes. Start on known-dry material for a baseline, then work in a grid toward the stain and write the numbers down.
The map shows the true edge of what's wet and whether a panel that looks fine is soaked behind the surface. If the numbers are dry, don't cut that wall.
Step 5: Look inside the cavity with a camera, not a saw
Most cabinets have a way in that isn't a saw: a removable panel, a drawer, a vent, or the gap under a toe-kick. Feed a borescope through it first, like the DEPSTECH WF028 wireless endoscope. You can see the back of a shower valve and the PEX behind a sink through a hole that's already there. A mirror and flashlight work on shallower runs. Get eyes on the fitting before you decide the wall comes apart.
The usual culprits I find
- A hand-tightened fitting behind the toilet or under a sink that vibrated loose on the road.
- The water heater drain plug, bypass valves, or inlet/outlet fittings — especially after de-winterizing.
- The pump strainer bowl or a pump fitting that seeps only under pressure.
- A cracked P-trap, loose slip nut, or shower drain that only leaks when that fixture drains.
- The city-water inlet check valve, which can let pressure bleed back or let the tank overfill.
- A toilet water valve that drips down the back where nobody looks until the floor goes soft.
Leave yourself an early warning
Once it's fixed and dry, put a water alarm at the low points — under the bathroom sink, by the water heater, and in the wet bay. A kit like the Govee water leak detector with gateway alerts you when a drip comes back, instead of letting it soak the floor for a month. Cheap insurance, not a repair.
When you do have to open something
Sometimes you do. But open the smallest, most repairable place first: from underneath, through a cabinet back, or a panel meant to come off. Photograph it first, shut the water off, and cut access you can put back cleanly. Diagnosis first, demolition last.
Do it in order — pump test, isolate the system, paper towels, moisture map, camera — and most plumbing leaks give themselves up without a single new hole in your RV.
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