What you're concerned with is the tongue weight of the trailer times the leverage created by the distance from the hitch to the rear axle. It isn't likely to be a problem, but I would pull it across a scale with the trailer attached and get an axle by axle weight just to be sure.
Your towing capacity will likely be determined by your transmission's gross weight rating. As an example, Allison rates the AT545 at 30,000 gross. If you go by the book, most busses with a 545 shouldn't tow much weight at all.
The MT643 is rated for something like 45,000 gross in a school bus (and much more in other applications) so towing an enclosed trailer with a heavy vehicle inside wouldn't be a problem - weight wise, anyway.
If you're towing (no matter what transmission you have) I'd strongly suggest installing a dedicated transmission cooler, a transmission oil temp gauge, and switching to synthetic transmission fluid.