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Whenever I work on my own rig I add shut-offs to the lines. It just makes things easier should a problem arise in the future and if something should go wrong with one fixture or line while camping, you can shut that one off and still go on using your fresh water system.
Maybe the fresh water tank's drain plug? For reference, my tank has two drain plugs, one with a valve external to the tank that I can manually open. The second one is on the belly of the tank that I have to access by removing a belly panel and pulling away the insulation. (The tank hangs above a thin metal belly enclosure with insulation.) Had the OE belly plug rot and leak where a run to the local hardware store for a simple plastic thread-in sewer / clean-out plug (IIRC 1-1/2") was the cure.
It's definitely the drain plug. I put a 3" extension in and the new fitting wouldn't ever tighten up in the tanks threads. I'm thinking that the threads are bad. I tried to put the original plug back in this weekend and it was very difficult to get it to start on the threads. I think I need to find a tap that fits the threads and redo them just to be on the safe side.
One more question: Does the drain plug on a fresh water tank have an NPT thread type? A 1 1/2" NPT tap is incredibly expensive. But it's a lot less expensive than a new tank!
Am coming up short of ideas less than caulk (which I am not recommending). But, the question that comes to mind is that, if the tank's threads are shot, just how long until another location on the tank turns into an issue. Naturally, am not seeing what you are seeing to help better.
And just checking, the plugs are not sealing even with teflon tape?
In my trailer, it was a clear case of the plug's loss of integrity (the threads on the plug started to flake-off) and the tank was still solid, so it was an easy fix (once I stopped trying to save the plug).
When I put in the plug I may have used a minimal amount of teflon tape, or at the very least not much. But even without teflon tape the extension fitting should have come to a point where it wouldn't turn any more because the fitting had reached the end of its threads.
I should disclose that the new fitting was ABS plastic not PVC. The old plug (which now has problems getting started) was PVC. At the store the threads for both ABS and PVC seemed to be exactly the same. But maybe not?
My goal of running a tap on the threads is mostly just to clean up the threads to make sure they are as good as they can be given the circumstances.
I tried to get a picture of the threads with my USB inspection camera. I'm still learning how to use the camera and the photo isn't that great.
Couple thoughts. The treads are the same and interchangeable. Since you are trying to chase tank threads, which are soft, you should be able to use a 1 1/2" pipe nipple which is much cheaper.
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