several things to check
one is operation of the heater valve, make sure it is open
pop the front hatch, find the back of your heater core
valve is in the hot water line
ALSO some coaches have a hot water cut off valve near the engine
that come into play when using the A/C its electric, also look for a hotwater shut off or manual bypass valve
if this valve does not open you will get very little heat from the heater core
our cumins diesel is smaller than your engine, and our heater is very toasty up front
some RVs your size have (2) furnaces, check for that and IF so it might not be on