how long has your bus been parked?
could be the brake pads and drums are frozen?
try rocking it back and forth?
drive throttle reverse throttle back and forth and or ( not really recommended) but spread wheel chocks apart about 6" or so on either end of the tires and not just one set.
have someone you trust try rocking the bus while you are underneath with a hammer to beat around each drum do try to brake them free.