People that manage fleets will suggest gas over diesel for private use if it will do the job, and the annual miles are low. Read up on some fleet management forums to get the real skinny on vehicle costs, gas vs diesel.
Ford still offers the v10 in their f650 and 750 medium duty trucks that gvw up to 33k, I think that says a lot for the engine.
I am on my second v10 f250 and am very happy with them. I don't read much against them except fuel mileage.
That said, the buses you're looking at have depreciated out so far the purchase price savings of gas is small if any.
Make sure the rear frame rails of the transit bus are up to the task of towing, if you aren't able to fabricate them yourself.
Have you considered looking for a school bus geared for the highway, haulding the rzr's in the back with the camper in the front?