Winnebago Sightseer 33C RV Propane Furnace?

Although I have a different model than you I also notice that the furnace wasn't pumping out the volume of heated air that I would expect.

We do a lot of winter camping and this was an issue.. the furnace ran a long time to heat the coach.

Here is what I found.
On my coach (different model than yours) there was only one return vent in the rear of the coach. The problem is that this return opening in the floor is the same size as the all of the heater outlet vents.
While there was only return vent.. there are 6 heated outlet vents. The furnace couldn't get enough air into it. It kept kicking off on the high limit switch.

I pulled the return grate off of the floor and inspected the heater area and found that I could easily double the size of the return vent.

I went to the local home store and found a metal floor grate that looked good and was the right size.. cut an opening in the floor to the return area of the furnace and installed the new vent cover.

Worked like a charm.. the furnace no longer kicks off on the high limit. Air volume on all of the vents have increased.

I also had a forward basement area that didn't have any water or sewer lines in it.. so I closed off the heat vent to that area.

Hope this helps.