Tom,
There is almost no way that the heater core in your dash A/C has total internal blockage. The highest probability for no heat from the dash system is either a malfunctioning heater control valve or a bypassed heater core. The only way to determine your problem is to check the heater hoses for a bypass because of a leaking heater core, and if there is no bypass, check the control valve.
If the heater core is not bypassed and the control valve is operating properly (hoses to core are hot), the temperature door in your A/C system is not operating properly. The temperature door is a door that directs air flow through either the evaporator core or heater core or a combination of the two. That door is how the dash A/C system controls how cold or hot the air is that it is delivering to the coach interior.
Since you did not say what model coach you have, I have a little difficulty in telling you exactly how to check the control valve and temperature door systems, but they are both basic automotive A/C systems. So it should be straight forward if you know anything about automotive A/C systems.
Gerald
