I am very happy that I bought a much lower cost gas RV than a diesel one. The fuel cost is really been not as significant of a cost over the past 20 years as the insurance, oil changes, and other maintenance items.
My Bounder motorhome cost $67,000 back in 1996. I was looking at a new 2004 DP Expedition motorhome, with a 167,000 MSRP and the dealers at the FMCA show said I could buy it now for $118,000, so there is significant mark up in the pricing, and this unit was not on their 'floor' so they had not needed to buy the unit that I was thinking of buying, and they could sell it to me with very little out of pocket costs to the dealership, thus the very low mark up at the FMCA rally.
My payment was about $607 for the gas RV, while the DP would have been in the $1,000 range.
There is no way that my total insurance cost or payment savings would have made up for saving $100 a trip with the DP motorhome.
If you 'require' a 35 - 43' long RV, then a larger diesel engine is also a requirement. But I have a 30' gas RV, and my gas engine (460" V8) is powerful enough to get it down the road. The V10 Ford and 8.1L V8 GMC motorhomes can go up to about 36' and 20,000 pounds while not being under powered. But beyond those limits, they become underpowered, and I would say go with something diesel.
Diesel will be much more expensive overall. I have about 50,000 miles on my RV. I get about 7 MPG, so would have used something like 7,000 gallons of gas. Should I have bought a DP, I would get say 9 MPG and used about 6,000 gallons? Over 20 years, that is barely any fuel savings, while the $400 more per month payment and more expensive insurance would have reduced any fuel savings.. .
GOod luck,
Fred.