If it comes down to which is better built, a skoolie is better than a shuttle bus. But that doesn't mean a a shuttle is all that bad. If you find a soild one, it will do you just fine.
If you are used to a Ford van, then you'll have no problem with a similar Ford shuttle bus. (my wife drive ours with no reservations) The front is exactaIly the same-its just bigger in your mirrors.
And many of them came with a 7.3 diesel.
As far as mileage, the best you'll get is low double digits.
I get 10mpg-with a heavy bus towing a trailer in the hills.