Just a couple of comments on MPG
I now have a 2005 Cruise Master, 36' on a workhorse W-22 chassis. I have added the short Banks exhaust and a KN air filter. My TOAD weighs 3500 lbs. I average 8.7 MPG, with my best ever 9.3 MPG.
My prior MH ( 30' ) had a Chev. 454 on a P-32 chassis. I had the full Banks exhaust, Hyper-Tech chip, and the Jackobs performance ignition. My TOAD then was a SUV at 3800 lbs. I averaged 10 MPG.
Some additions do work, but a lot depends on how one drives. When I added the Hyper-Tech chip, it boosted performance and if not carefull I would drop in mileage.
It's my feeling that the best bang for the buck, is the Banks exhaust system.