All depends on how you plan to travel and where. Will you tow any thing? Travel in mountains? Diesel will get better mpg but gas is cheaper. Diesel cost more for oil change and may be harder to find places to repair in case of breakdown. Diesel if you plan on driving 50 k a year. Most rv averave about 6k a year. A gas engine will last for 200k easy now if you dont overrev and keep oil and filter changed. Dont know about class c but class A diesel tend to ride better. I have owned a class A gas v10 ford since 02 and no driveline problems. Gas rv will be around 10k cheaper. All depends on you
I'm sure you'll find diesel rigs are more expensive to buy, more expensive to maintain, and about as expensive on cents per mile; more expensive fuel but better mpg. Diesels usually have better pulling power and ratings, but are usually installed in heavier rigs that need more power. If you're pulling a big toad or trailer, you'll likely need a diesel.