I have a 38 foot Class A and the other day it was 22 outside, turn on the furnace in 20 minutes it was 65, wife said turn it off, the back bedroom get the hottest, sometime we only use the heat strip in the overhead A/C units and it keep us warm. you need to check something, that to long for a furnace to run and not heat up your unit.
Your furnace may be cycleing on and off. The fan keeps running but the fire goes on and off. Not real noticable inside the unit. Temp coming out of the floor duct will vary from flame on, 170f to flame off 110f. This is caused from the limit switch cycleing on and off. If your ear is right next to the furnace you can hear the flame come on and off.
1994 27sl Alpenlite, 2001 Dodge Cummins auto 3.54 stock