Class A to a Fifth wheel......What to expect on MPG???

I won't try to talk you out of the switch but as far as using less fuel is concerned, something to ponder: Diesels cost more to purchase and to service and the fuel can cost as much as 60, 70 cents or more above gas. Also, fuel isn't really the biggest cost of RVing. It is usually the cost (depreciation) of the RV and the tow vehicle which will be huge in your case.