The AC circuit is the easiest of the two to troubleshoot. It is TOTALLY independent from the DC/Gas part of the heater and will heat water with the GAS off as long as the camper is plugged in to AC power and the main circuit breaker is closed in the panel. It has NOTHING to do with the "control board" which is used to ignite the gas.
The ON/OFF switch is "typically" on the outside of the camper (though some use a separate switch inside near the one with the ignition light).
There is also a high limit reset button under a rubber seal to reset the high limit switch should it need to be reset.
The heating element could be bad due to having the AC switch on without water in the tank.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW