Well you probably won't find a skoolie with much more power than a 5.9 in a TC2000... at least from what I've seen. Maybe there's something wrong with your engine, maybe a partly clogged fuel filter or something. My bus is no race car but it is compared to some other bus's.
As far as towing 15,000lbs with a hitch... (maybe this is a little off topic and you already have this part figured out) Aren't pull behind hitches rated at 10,000lbs max? I've never seen a hitch or a hitch ball rated for 15,000lbs unless you're talking about a pintle hitch. Like one of these...
Anyway if you're asking if anyone here has any experience towing a trailer that heavy with a transit/tour bus you might not ever get an answer. I think all but 1 or 2 of the bus owners on here have skoolies. Which as you already know have less power than a full size bus.
Jason did tow an overloaded trailer all the way to burning man last year from Michigan - about 5,000 miles total I think he said it weighed something like 12,000lbs - he may chime in but again that's a skoolie towing not a trasnit bus.
Good luck with your search.