JohnS has a Super C, even though it's under 30-feet. F550, V10 Gas or 6.7 Diesel confirms that. Different Animal. The Ford Based Class C's on E-Series chassis have about as many disadvantages in Diesel as we hear about in Gas.
Ford isn't doing Diesel in E-Series anymore, but Chevy is, in their recently (last few model years) 4500 Cutaway/Class C chassis. I think... I know it's available in a Box Truck Chassis. It's billed as Duramax/Allison. If it's the pickup truck power plant, all tweaked up to max HP, could be very interesting. If it's de-tuned like many of the diesels used in pickups, are when installed in anything else, then...? Ho Hum...?
We've been to Georgia/SC mountains with our 31' (signature) towing a Corolla. Sure it could use more power, but it downshifted and we ran along just fine. Could probably pass some of the diesels, but that's not the point and it'd be screaming RPM's to do it.
Our first Class C was smaller, had a carbureted 460. Towing a smaller Toyota, we'd leave diesels behind on the grades.
FTD, I agree that gas stations are a PITA. The big turning circle of a stretched E450 makes it even worse. And there isn't gas on most of the truck lanes anymore. Used to be a lot of gas powered box trucks and moving trucks, but I guess they're all gone. I really wish more travel centers would put in "RV Lanes" and of those, they wouldn't point them At The Building like virtually all are.