My wife complained the water was too hot. Couldn't find a way to adjust it, so my Dad (older, wiser, camping longer) insisted he would show me how to adjust it, just as he was able to on all the trailers he previously owned. Nope, he could not find a thermostat either.
Come to find out.....they make them with a set tempature (HOT), so you add cold---thus minimizing the usage of limited availability of HOT water.
Have you check the burner???? Can it be clogged, limiting the amount of gas burning? Not getting to that set HOT temperature?
What is the tempature of the hot water at the tap?
What about an outside shower or other outside water valve? Is it closed at the diverter, not the shower head? Could be mixing cold with hot...