11 mpg with a diesel and 7 with gasser are both pretty ambitious. It's not unheard of mileage, but I would suggest you check out the postings on the mileage poll on this site. 11mpg probably means a 5.9 powered TC2K traveling at 55 mph with an MT643.
The maintenance costs really are kind of a wash I think. Yes, diesel stuff is more expensive and you have to do stuff like change fuel filters once per year or so, occasionally add fuel treatment depending on the fuel you're getting (ULSD can be tough on older engines), etc. But step back and take a good look at things. A gasser might get 3000 miles on an oil change costing $30. My diesel gets 6000 under severe service at $60 for an oil change. A gasser will have nickel and dime stuff like dizzy cap, rotor, plugs, wires, etc. A diesel GENERALLY will not have those issues.
I was in the same boat when I bought mine. I know my way around a Chevy big block, but diesels were beyond me. After buying my diesel I don't know that I would ever go back. Maintenance is easier than you might think and if you get a good one you won't need to be worrying about major stuff. If you do it's probably time to consider a new bus, gas or diesel.
This isn't a knock on gassers at all. They are the hotrods of the bus world and parts for a 392 or 366 are way easier to find than stuff for something like my 6.6 Ford Brazilian. You need to make an educated choice for yourself, but don't let maintenance be a major issue. Think of it like modern engines versus those of yesteryear. Carbs are easier to maintain than fuel injection, but they also require it a lot more than fuel injection. Just some food for thought.