RV furnace ducting is very basic, even on the most expensive class-A DPs.
It's rarely ever balanced as a home system would be with different sized ducts, air valves, and vents so that the air flow is very similar from all vents. In an RV it's simply about distribution, (just getting the air there).
Unless there is some physical object blocking the air flow, the flow restriction results from the bends and duct length to the farthest vents. The closer vents, (to the blower fan), blow more air than the farthest vents.
If you really want to balance the air flow throughout the system, (it's very easy), you restrict the vents closest to the fan blower motor, (with some foil tape behind the vent grills). That will increase the air flow to the farthest duct vents.
If that doesn't work, then there is something physically blocking the air in the ducting. It's likely that the air is being blocked by a rat, family of mice, a bat, squirrel, snake, bunny rabbit, or something completely boring like leftover assembly/manufacturing material.
And, you don't want to haphazardly stick anything damaging in the ducts to randomly probe for blockage as the RV HVAC duct hoses are typically manufactured from ridiculous fine delicate metal/plastic gossamer that makes gold -leaf seem sturdy. In the walls/floors/ceilings you will find some sort of soft high-tech insulating foam, (usually Styrofoam) channels at acute and right angles. So, if you do need to probe, do so very gently with something very soft and flexible.