Heater won't blow hot in my coleman Seapine

Sounds like either the heater core is plugged or the hot water valve isn't working

You know you are setting yourself up for a fall when you say you checked everything.
Is the blend door working under the dash ?
It controls the vent temp.

you might want to check the module under the glovebox

Does it leak any antifreeze on the floorboard or anywhere? Common problem with the jeep cherokee that the heater or A/C will cause it to leak and had to replace the whole unit.

Sounds like the automatic fan control is not working it cuts in on temp rise . It may be dirty on the back of it as you said you had a smell of dust burn

You can buy one directly from Coleman that will fit your heater. Use the old part # and they will find a comparable one for you. 800-835-3278

try back flushing your heater core locate the 2 hoses that go into the firewall start your motor and feel for a hot or warm one thats the inlet to the heater core your gonna have to remove both hoses and using your garden hose flush it out through the hose that was not hot.

You likely have a restriction in the heater coolant hose circuit - perhaps a coolant control valve not fully opening, or a kinked hose.
Insufficient coolant volume is flowing to keep the heater core hot.
Try back flushing the heater core system with a garden hose to check flow.

First check the coolant level in the radiator. Check your heater core hoses on the firewall underneath the hood. Let the car come up to temp. and feel the two hoses. If only one of them is getting hot you may have bad heater core or a bad heater control valve.