I do recommend a brass check valve, however. Stay away from the lesser-expensive plastic check valves. They have a history of distorting and for chronic failure when positioned near a heat source, (the hot water in the heater). Some plastic check valves, (or any type of valve for that matter), can fail and remain permanently open OR permanently closed due to the deterioration of the internal seals. Your best bet is with a brass check valve.
Hmm...ever thought about becoming an RV service technician? It seems you have a good foundation of correct troubleshooting skills! Congrats!