The brass fitting coming out of the heater in the above picture, may be the check valve. It may be stuck, and need to be replaced. You can do it yourself. by draining the heater down, remove the line and then remove the valve. There are a couple of different ones, so take it with you to the RV parts store. It is just a spring loaded seat, so you may be able to push gently down and unstick it.
I always carry an extra, because I paid over $200.00 to have one installed in Davenport Iowa.
The valve is from $5.00 or $ 10.00