I picked up a 2021 261BH last spring and sure enough first time running the heat found the Bathroom to heat up to over 100 F and no heat in the bedroom. After checking and I finding this thread I started digging in to the problem. First I would like to thank rraley for posting the vent drawing. What I found was a different set up and other issues. I updated the drawing supplied by rraley to show the layout of duct work on my trailer.
First off my trailer has 5 ducts coming off the furnace. 4 x 4" ducts, run to the bathroom, to the front bedroom under the belly, to the dinette table and one running to the under belly as well as another 2" duct running under the belly to the other side of the table. The Bedroom duct was a complete mess as they ran the 4" duct over the slide out rails between the rails and floor (about 2.5" space) and it was ripped open as well as the duct had a cut in the wall between the furnace and bathroom as it was too short and they just stretched it until it ripped. The 4" duct run to the bathroom moved the most air (heat) as it was the shortest run.
To correct the above I installed a damper at the furnace on the bathroom line, at fully closed it was still blowing a good amount of air (heat) to the bathroom. I removed the duct running over the slide out rails and installed semi-ridge duct bent into a oval shape to clear the rails but still have the volume of the 4" duct. These modifications helped in above freezing temperatures 40F, but we used the trailer for a few days during the holidays at 5F over night and the furnace was struggling and the trailer had many cold spots, mainly the bedroom as all the duct work under the floor was being cooled too much by the low temperatures to provide a large temperature rise to heat the room.
I'm thinking this summer to make some major modifications to the duct work and try run the majority of the duct in the heated area. For example in the bathroom the water lines to the vanity run in a covered box in between the shower and the vanity, I'm thinking to T off the bathroom duct and run a line to the vanity in the box in to mount a vent to the living area between the bathroom wall and the slide out to add more heat to the living area. As well maybe run a 4" duct along the same route as the first vent under the table then add a box in between the bench's to add another 4" vent into the shoe storage rack to heat the front by the main door. I could use the belly vent line to supply this run and then use the current 2" duct to heat the under belly. When camping in low temperatures we dry camp anyway as it is too much to re-winterize the trailer for a few days of use anyway.
In short this trailer is not cold weather ready as they suggest. We found the front wall to be very cold and they claim that the front is R40 like the roof. As for the claim that the underbelly is R11 I would like to know how a 1/4" plastic corrugated sheet has R11 rating? Unless they forgot to install the insulation in the underbelly as my trailer has nothing other then the corrugated sheet on the underbelly.
If anyone has done any modifications to the duct work I would appreciate details on what was done and how it worked out for you. Not looking to make this trailer a -20F rig just would like to extend the use of it during the "warmer" weather AKA above 0F. Thanks.