This is from Gary Brinck – RVForum.net
Poor Air Flow from Duct Outlets
This is often caused by crimped ducts or excessive length ducts that have extra loops that slow down air flow.
Check as much of the ducting as you can reach and straighten loops (removing excess if possible) and eliminate kinks and crimps as much as possible.
Also check the outlets themselves – many are adjustable and may be partially closed.
Take the outlet off and make sure it actually opens – broken ones are not uncommon.
Gary Brinck – RVForum.net
Possibly blower fan is loose on the motor shaft
If the fan is working then it would appear to be closed registers or restricted ducts.
From my limited experience, access to the furnace output to ducting is behind the cold air return.
Is this a new to you RV? If not was the furnace working properly previously? What is the model number of the furnace?
(Is it behind the white plastic panel from the outside. I can't figure out how those clips work to get that panel off. Do you squeeze those black clips in???
I believe the white plastic panel is the vent cover for the refrigerator.