So I was looking for some way to refill the water inside the RV after it shuts off in the world and I decided to mess around a bit with the debug menu mod and found out a few things:
- The inside of the RV is basically a house that exists in it's own separate map, and on the ceiling there is a bunch of large rain collector barrels and a generator;
- The barrels are not hooked up to the stuff in the RV (the sinks, toilet, etc.), so you'll have to manually hook them up with a pipe wrench;
- The generator is seemingly infinite so energy isn't a problem;
- Even if the inside is technically a different map, it rains in sync with the overworld, so you'll be collecting rain water passively;

Also a few issues:
- In some variations of the interior, the generator is bugged and doesn't work, only way I found to fix that is disconnecting it and reconnecting it manually on the ceiling, which you can only do with debug mode;
- You can't get rid of corpses inside the RV, either wait for them to rot away and despawn or debug mode again, also there's a variation of the interior that spawns with zombies so good luck with that;
- You can only do plumbing on the second floor.

This is an amazing mod and these are some kinda obvious things, but they're pretty annoying and I haven't seen the author nor any comment addressing them, so might as well throw this out there so it can maybe help someone.