This project fixes the problem of poor furnace air circulation in the RT Independent model when the rear bed is down in the bed position.
Step 1 is to determine how many sq. in. opening is needed.
The furnace air return has 8 one half by three inch slots that total 24 sq. in. or a 4"x6" hole.
The Suburban furnace register has 3 slots 1/2 in. by 8 in. which totals 12. sq in. to provide return air.
Conclusion: try for 24 sq. in. and anything over 12 sq. in. should work.
There is a 11 in. space between the furnace and the tub/shower that needs holes in both sides for the new air passageway. (Future project idea: use some of this space for extra storage)
After the hole is cut in the tub wall it is evident that there is plenty of clearance around the tub for sufficient air flow.
The next step is to cut the hole in the other partition and then block off the old return air path.
I didn't see any clearance specs.but I held my hand on the furnace after it ran for 20 minutes and it was not hot to the touch so I dared to temporarily use wood blockers. I will change to metal now that I know this works.
This is where the new return air grill will be.
The hole is cut and then the new register is applied.
It is a 6 by 10 inch flat return air register, - plenty big but that is the smallest they had at Home Cheapo. They didn't have brown so I'll have to paint it.
(Added benefit: A minute or two and a screwdriver makes this a good hiding place for an envelope of valuables)
The last picture shows a butane lighter flame drawing towards the opening with the furnace running.
Before this improvement the furnace would run for 5 to 10 minutes before any noticable heat could be felt above the bed but now it is almost immediate, a very noticable improvement!
"Thanks Markopolo for adding the pictures for me"