Originally Posted by ganmercy
it is an Atwood 6gal
The starting place is to check the burner area for critters and mud daubers nests. Check the flue tube, that's where the flame inters into the heater to heat the water. Could be a nest or critter in there.
If all that is clean and clear have someone check the gas pressure from the regulator. Give it all a good look over and then check the wiring on the control board.
This will give you the places to start.