I had the same problem and found the ECO had shot.
Mine is a Atwood gas/electric 10 gal model and the first time I used both 120v and gas at the same time the ECO blew.
I called Atwood and asked if there was a higher temp rated ECO avalible and they,of course, said send the water heater back in or take it to a dealer.
Their stance is "make as much as you can off the consumer".
If I had done what they said I would have been out $150+ and the darn ECO would just blow again as soon as I used the 120/gas combo 'cause they only make them to blow at 180 degrees plus or minus 20 DEGREES.
Flame me or not........I bypassed the thing and it has been working great for 5 years..........after all it does have a pressure valve on it.