Don't know about RV furnaces, but some furnaces have a limit switch where the heat exchanger has to be up to a certain temp or the fan stops until temp comes back up. If for instance you were just turning the furnace on in a real cold house, the fan would stop momentarily every time that real cold are on the intake side lowered the heat exchanger temperature to much; until the heat exchanger came back up to operating temp, then the fan would kick back on. So if your RV furnace has the same type of operation, this could be normal. If however the house ain't that cold, could be some other problem.
