[bstark]

From what information being provided; furnace running but not heating unit enough to trigger thermostat shut-down, I would suspect the problem is with the furnace and it's supply ductwork to trailer. There really shouldn't be any problem with the air coming from the furnace being warm as these furnaces are at least 35,000BTU's rated by D/T to produce 42,000BTU's through the use of a redesigned (tapered) plenum duct on the rear of the furnace. A run time of a few minutes should result in you noticing very warm air from floor ducts in living room/kitchen area.

A furnace that is running but not producing heated air that is almost too warm to stand with a bare foot on the floor grills is not functioning correctly and should be checked out viv-a-vis burner performance.

I would start my investigation by checking the ductwork right at the rear end of the furnace and if it's very hot there, following the plastic ducting to their respective connections to the metal floor ducting. If there is not significant heat right at the rear of the furnace you have a burner/gas supply problem.