gas or diesel?

Depends on so much. Didn't notice if comparing Class As only, or not. A diesel pusher will be quieter than a gas Class A, obviously. However, we have a V-10 in our F-550 based Super C and I can't hear it (that's with Banks exhaust too). A diesel in a van cutawy Class C is loud. A diesel in our rig would be louder. If you're set on a 40' rig, then I'd say diesel pusher, without a doubt. If looking at something smaller, then the V-10 may be a good choice (the added price for diesel at the pump may come close to zeroing out any mileage gains, insofar as your wallet is concerned). Apples to apples, I know the exact same rig to what I have, but with the plagued 6.4 diesel, only gets .5 to 1 mpg better than my V-10. The high-revving three valve V-10 has more than enough torque to get the job done, for us, even when pulling a heavy race trailer. The 3 valve V-10 is a very reliable engine. But again, if going with a 40' A, then diesel by all means (unless it's a Cat ;-) ).