There could be one or two hot air ducts loose from the heat exchanger chamber. Look for the cold air return, it will be approx 12" x 12" or larger. Remove the panel with a #2 phillips or square screwdriver and you should be able to see the ducts connected to the hot air chamber. Sometimes it doesn't take much jostling for them to come loose.
Richard & Jill
2014 Flagstaff 832IKBS Classic Super Lite
2018 Silverado 1500 Crew Cab Z71 4WD All Star Edition
Camping since 1989
Car Shredder Op/Tech, Scrap Metal Recycling - retired
I deserve hazard pay when camping with my relatives.