First step would be to look for a relay in the system that is possibly stuck.
If no luck finding a relay , have a good look around for a company name on the A/C heater box housing . Denso & Evans are common names of heater and A/C systems for Class A , motor homes. If you can find a name then a visit to the company web sites will provide more info on tracking the problem down.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.