No direct experience but that's a pretty big leap. There are A LOT more capable class C or other RVs that have been made since 1990.
The things that would worry me with something that large which may or may not be a problem for you:
General driving - fitting into campgrounds on small roads, etc.
Storage - do you have room at your house to store it?
Service - you're not likely to just have the tools handy to change a flat - let alone do anything else
Moderate/major repairs - if you have something go wrong you could be talking about some MAJOR money to work on it. For instance how much is a head gasket job on a Provost bus? How about getting it towed to a garage?
A new bus chassis is somewhere north of a couple hundred grand which means anything in the cost ballpark that you and I common men could afford would likely be very high mileage and therefore more likely to have problems.
I'm with you on considering it and it would be great when rolling down the road. I just wouldn't want to be the one footing the bill when something goes wrong.