DFord wrote:
I have no idea what kind of mixing valve you have on your shower but I'd suspect some debris in the line leading to the how water side. The easiest way I know of to make sure would be to drain the pressure on your system and remove the valve for inspection. Once the valve is removed, you could put a plug in the cold water line and a hose on the hot water side and pressurize the system and see what kind of flow is available to the shower. If you get a full stream with plenty of pressure, the problem is in the mixing valve and it would need to be backflushed, disassembled for inspection or replaced.
I agree it is very likely a mixing valve problem. Not only could it be debris in the lines it could also be the valve itself has a defective anti- scald component. Nothing special about RV plumbing, so any plumber should be able to fix the issue if you aren't comfortable doing it.
