A few years ago I had a Wildwood and it had an Atwood gas/electric water heater.
To use it with gas there was a switch on the control panel, I think, and for electric there was an on/off switch on the water heater itself.
Like the others said, be sure there is water in it. You can check this by gently lifting the pressure relief valve handle with the water connected and on. If the tank is full you will get water out of the valve, if not you will get air. If you get air just hold the valve open until you get water.
2013 Coachman Mirada 29DSSE