Had that problem in my Shasta Travel Trailer, Ducts poorly routed, flex duct crushed, too sharp of bends etc. I jerked all of the Flex Duct out except for the 2" under the floor to the bathroom. Replaced it all with 4" round Metal Ducting and fittings with fiberglass insulation. Furnace quit cycling on the over Temp Safety, the cabinets around the furnace cooled down so the food stored under there was not getting way above the ambient temp in the living quarters. I would do it again if I ever purchased another unit with the Flex Duct in it. Motor Home already has metal ducting so I have no issues with that furnace.
GSMC(SW) U.S. Navy (RETIRED) 1967-10-03 to 1993-11-01
1995 Gulf Stream Scenic Cruiser, Cummins B5.9, Allison MD3060, Toad: 2005 Focus ZXW SES Wagon