You've gotten good advice on checking the vacuum pump, vacuum hoses, heater valve, and the resistor for fan speed.
My coach is similar, and I've never gotten decent heat out of the dash heat. It will work fairly well at idle, but not at highway speed.
I've been advised to turn off the valve to the motoraid water feature (coach hot water for sink/shower) to force more heated coolant forward to the heater core, but I haven't tried that yet.
It is a long way from the engine in the rear of the coach to the heater core in the front of the coach.
Good luck, and let us know how you get it working.
Fred