I would start by checking for 115V at the water heater. You should have it there when the switch is turned on. If you don't have it you track back towards the breaker panel. If you do have it, then you probably have a burned out element. The gas portion of it gets a little more complicated so I would start with the 115V side of it.
You also need to determine if all of the 12V stuff either all works or doesn't work at the same time. You could have a converter issue, but if you did, and it cuts out, then nothing that works on 12V will operate. That would be the overhead lights, water pump, water heater running on propane would use 12V for the control and igniter. Furnace also uses 12V for the fan and control circuits, nothing on 115V. This should get you started.
2013 F250 XLT
I now have a Keystone Outback, but I try to help when I can.
Bitter Gun Owner