It's not uncommon for the heater control valve to be screwed up in some way. Either the valve is stuck or the cable is broken. Some heaters have a bypass and required the heater valve to be all the way open before you get heat.
I had one gasser where a previous owner installed a manual heater valve. To get more heat, you needed to stop, open the front and twist the valve.
In both my coaches I replaced the heater valve with an electronic one from Thermotion. If you have a 2 port valve and a bypass, get a four port valve from Thermotion and replace the bypass with the valve.