First off, the most important statement that has yet to be made is that not all diesels are created equal. While a DP usually means higher end, that is not a universal statement, and there is some overlap where a higher end gas motorhome may be built considerably better than a low end diesel.
Generally speaking, I would lean strongly toward going older and higher quality, regardless of the fuel type. What ever your choice in quality is, you are married to it, with divorce being very expensive. 10 year old quality is still quality. Brand new junk is always junk, even when its new and looks fancy.