That looks like it will work really well for the Versatile model Roadtrek, however my Independent model has the tub/shower jutting into the aisle right where the warm air terminates.
I looked at mine today and decided to cut a return air register in the aisle tub surround facing the front of the van and then cut corresponding size holes under the bed support cabinets back to the furnace. I appears to have enough space for the volumn of air needed. I'll have to block off the former return air path so the furnace fan will now draw the unheated air from the middle of the van. The problem with the factory setup is that when the rear bed is made up, the furnace draws the unheated air from the same place as the heated air is sent resulting in a hot spot under the bed in a short cycle and letting whatever heat convects out of that area heat the rest of the van. I'll take before and after pics to share, although I don't hear of too many Independent models around that might need this info.
Don 92 RT Independent