Sounds almost exactly like the situation I had... plenty of toasty air in the utilities space but in the coach, not so much. Would come out like gangbusters from under the sink when the cabinet doors were opened.
It turned out that there were two problems at the furnace. First, the rear panel on the plenum was open on one side so the heated air was just circulating in the utility space. The little push-and-turn 'nut' had loosened and fallen out - didn't find it until after I had closed it back up with a self-tapping screw. Second was that one of the duct bayonet fittings had come partially loose. Just the right combination of twist that it rotated itself out so I redid the connection with a different twist.
In my case, I had to remove two or three carpeted panels in the pass-through storage to get to the furnace.
2015 Rockwood Signature UltraLite 8282WS Platinum, GY Marathon LRD, TST 507RV TPMS
2005 GMC 2500HD CCSB D/A, Curt E16, Prodigy P2, Garmin RV760LMT w/BC-20 b/u cam