Regarding 4. above: Will not ignite... What won't ignite? The Gas Flame should not burn in Electric Mode. Or do you mean there's an Indicator Light and it doesn't glow in Electric Mode?
The only Atwood water heater I've had or worked on, was a simple Standing Pilot one, similar to the old home Gas Water Heater. It had an electric element that was a simple circuit. Power to Switch to Thermostat to Element.
OB's info above says there's a relay and he's The Man when it comes to looking this stuff up. I didn't see a Model Number so I haven't tried to look up a manual...
But, at the end of the day, the 12-VDC side has to have what the phone company used to call "battery and ground." And Drew would say 120-VAC has to have "hot and neutral." Always be sure that Both have Both. Sometimes we spent all out time on the hot/positive side and don't consider the neutral/ground side.
That just happened to me. Water heater, 120-VAC side out. I was sure I'd burned the element out. Installed a new one, thing still didn't work. Turned out it was a failed junction box connection on the Neutral side. See if your Element has continuity. Power off, connections off, 10-15 Ohms on your DVOM. Re-connect, then Power ON, see if you have 120-VAC at the Element. If you have continuity in the Element, and 120-VAC on ONE side of it, the thing should heat. If you have NONE, it's trouble in power source, from Breaker to Controls, Thermostat, Relay, Wiring, Termal Safety. If you have 120-VAC on BOTH element terminals, then you have what just happened to me: Open HEUTRAL, that White 120-VAC wire.
That failure-in-the-junction-box thing didn't occur to me. Chris Bryant had to bail me out. But thinking of it, Water Heater's a heavy, persistent, load on wiring and connections. A long steady draw, not like making toast or a pot of coffee. Makes sense it can burn out those cheesy punch-down terminals.
