the wheels are pretty simple to remove in theory.....if they've been on there for 20 years they may prove to be a bit tricky in practice.
here's an important safety tip! Don't remove the wheel nuts all the way until after you break the wheel loose from where it's mounted.
jack the bus up, obviously you should make sure to follow general safety protocols, ie: use wheel chocks on the front wheels, jackstands or similiar device instead of a jack etc.
loosen all the wheel lug nuts, but leave each of them on the wheel stud a turn or two. There are little wedges that the lug nuts hold in place. These guys can pop out of there with a lot of force. Once you've loosened all of the lug nuts, get a sledge hammer and pound on the sidewall of the tire, or on the edge of the rim (where it meets the tire) until you break the rim free. Once it's breaks free, you can safely remove the lug nuts and the little steel wedges.
the outside wheel comes off pretty easily. the inner one isn't much more work. The buses i worked on had a spacer between the duel's but once the outside tire came off, the inner tire just needes some attitude adjustment and it followed suit.
i just uploaded a photo to help explain things.....