See none of the experts have responded in 10 days so here is first 2c worth to start. Away from the shop so I can't quote the manual correctly.
In the setup functions there is a adjustable range between cooling
. I have set mine to the max to prevent going into heating
mode when I turn the stat way up leaving the boat
. Suspect yours may be set that way.
If you are comfortable working on the unit and certain of proper control setup next steps would be voltage check at the reversing valve to verify stat is actually calling for heat and without compressor
operating attempting to drive rv solenoid to see if it is moving.