FIRST: Push the reset button on BOTH of the rubber discs on the water heater thermostat/reset buttons. If that doesn't work, then you need to confirm that you have 12 VDC AT THE CONTROL BOARD ON THE WATER HEATER. If you do, then troubleshoot the control board. If you don't, it's a wall switch problem, a electrical wiring problem or a connection problem. My first inclination would be to turn the wall switch to ON, walk outside and use a Voltmeter on the control board connections to make sure you have 12 VDC at the connections. If you do not have power, pull the wall switch and check the connections. If they are good and you have power through the switch, then troubleshoot the control board.
REMEMBER, YOU ALSO HAVE 120 VAC ON THE CONTROL BOARD AT THE WATER HEATER IF YOUR SHORE POWER PLUG IS PLUGGED IN, SO BE CAREFUL NOT TO GET ELECTROCUTED !!!!!
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS