I had a similar failure last year on a Suburban water heater. Worked on electric but the gas wouldnt fire off. Had the fault light on by the interior switch. There are two barrel shaped sides on the gas valve, mounted next to the water heater. The barrel shapes are the solenoid coils. I could feel one side vibrate as it was operating, but the other side didnt. Concluded that the gas valve wasnt opening. Of course the
local RV house had only one, but there are 2 sizes of gas line inlets and the one they had was the wrong size. I stopped at a parts house in Portland OR, to get the right size. Our family was on a camping trip so we were in the area, but Wow! the downtown Portland streets are narrow - glad I didnt have the trailer while I was navigating through there! Bought the valve, replaced it while at the campground. All was good after the replacement.