Welcome to the group.
I doubt you'll find a wiring diagram on that trailer. What brand Water Heater do you have? The two basic brands are Suburban and Atwood. If it is indeed a gas & elec., the Suburban switch is in the lower left corner behind the outside cover.
Atwood water heaters usually has a switch somewhere on the wall that looks like one of the photos below.
There is also a switch on the rear of the water tank. See switch # 26 below.