Is the water neater bypass on?
Tried it on gas and also electric. Burned out electric element (from being on with no water in the tank) is very common and a in-stock part at every RV dealer because of it.
Also see if you're getting warm water of the cold tap too. If the shower is turned off a the head it acts like a mixing valve. The outside shower is infamous for this and is usually locked in the lil' compartment.
-- Chuck