Is this a diesel pusher? If so, most dash air systems are operated by a vacuum system. In many, there's a small compressor that runs when you switch on the start key to run, no need to start it, it will be noisy enough to hear it. Some older RVs had a vacuum pump motor back near the engine. That also ran the cruise control.
Anyway, there's a vacuum operated mixer valve at the front of the RV that the dash air control operates and permits hot water to flow into the heater core. The air pipe connection to that valve often cracks, splits, falls off. So check that.
Here's what that valve looks like in DPs:
The small vacuum tube would connect to the top of that 'hat' shown here. You can check to see if heated water is making it up to the heater core by driving a while, then feeling the hoses that go to it. The input hose should be hot, and the output hose should be just as hot if the vacuum control is working.