I have to agree with Tabletop390's assessment on new RV's.
Some Friends that my Parents used to RV with have been burnt twice on two brand-new Class A's: one had warranty issue after warranty issue and it never got fixed right. They literally missed a season of using it. The dealer was 3 hours away and they put all the miles on it back and forth for warranty. They figured the only way out was to trade it (was a Workhorse, i.e. Blunder Motors chassis) for another new one, a Cummins pusher. Their drivetrain problems went away but had coach issues and wound up selling it after a couple of years.
They are now on their 3rd new Class A with a V10 and they finally got a good one.
If you read the RV forums, it is rather amusing to see the numerous problems and complaints about the workhorse chassis, especially because they are mostly die-hard gm owners.
It finally got so bad that they simply don't make them anymore.
My Dad had the workhorse 8.1 and it was a dog. The Newmar coach that was on it was something else though!
Definitely buy a used one: let someone else with more money and time on their hands sort out the bugs so you get a good unit. I put about 3 years of researching and looking at RV's until I focused on the exact one I wanted. I waited another couple of years for the price to come down and wound up with an awesome, trouble-free machine at 1/3rd the price of a new one. It only had 9,000 miles on it.
As for the diesel question: GET A CUMMINS. Cats are a 50-50 shot whether you'll get a good one or not. My Cousin has a Cat that's been good, but he says the fuel mileage is absolutely awful. There's another engine a Maxxforce or some such thing, it's apparently a variation of a Cat, and you want to stay away from them too, even if it's a good deal.