Mine has a similar issue with heating the bedroom a lot, and the living room a little. I pulled the basement apart over the weekend and see the following:
The living room is heated with 2-4" ducts that go to the island - I would guess that it goes to one plenum that feeds both ducts.
The Bedroom/Bathroom/basement are also fed with 2-4"ducts that feed the plenum. The plenum then feeds the two ducts and the vent to the basement.
The underfloor is fed with a 3 or 4" duct that goes under.
The plumbing station is fed with a 2" duct that is also tied into the underfloor system which is fed by a 4" duct. I was going to take pictures, but I forgot!
All of the returns feed into the area between the basement walls and the floor/stairwell ducts. They are open and are just empty draw spaces.
I am going to look at connecting two of the return registers to one of the 4" ducts that feed the bedroom/bathroom area. Since the heat is already heading upstairs by transfer, I think it would work better. Plus I like my bedroom cooler than the rest of the trailer. I'm going to look and see if I can put registers at one or more of the ducts and heat the entry way area. I believe that it would give enough space between the send and the return registers that I would be effective. I would cap off the other 4" opening at the bedroom/bathroom register.
Ct..