Most likely is the valve that allows hot water into the heater coil either is bad or does not have electricity to it. Fuse or heater switch could be the culprit. If you are getting cold air then fan and fan switch is good.
Good Luck and keep us posted please. "Q"
1999 Newmar, Mountain Aire 3768, V-10, CAI, Headers.
"Spending our kids inheritance one trip at a time"