Dick_B wrote:
Are you on campground water source ie. pump is not running? Does water come out of the cold faucets? Any chance the Heater bypass valve(s) have been changed?
I didn't know my water heater HAD a check valve!
Not all do. Some have them on the output line as part of the winterization system.
To OP. Hav you used the outside shower and did you leave the taps turned on to it but the head turned off. This causes mixing in the head assembly and subsequent lukewarm or cooler water in the rest of the unit.
Good luck. Please get back to us with the final fix.