Various reasons as noted, you need to buy what you want taking some consideration in on other owners reports. I was looking for a diesel found a couple but on 1 wife said too small, the other had been a live in model and showed it. The 6.0 in each performed well. The other we found was also used I liked it wife said looked too used interior wise! So you know you have to keep the chief happy. Ended up with a V10, an ok engine in my mind not the wonder beast many seem to claim. I also have a F350 with the 6.0 so know its limitations. V10 struggles mightily on long pulls at altitude, (denver area), has a tendency to rev highly when it starts to struggle to overcome its less torque. I added a mikes 5 star tuner, it did help civilize the engine transmission combo supposed to add some HP and Torque due to fine tuning and it does help.
As noted diesel c's are few and far between but everything in an rv is a compromise it seems. In a class A I would go for a pusher vs a front engine setup but thats me. I dont worry about 'resale', 'repayment' issues, thats not what rv is for. Just like your home its either a house or a home, houses are eventually for resale, home is where you will want to be/stay. My opinion and others may vary.