Stuck thermostat, huh, makes sense to me. I have gone through the same dam thing because of a thermostat. Especially if there is an air lock which sounds like there definitly is.
Now are the head gaskets gone? Only one way to find out. If there are not leaking internally it could be run back home with the rad caps loose, once the air lock is out.
I do think Don is on the right track though, the cooling system needs to be redone , especially if a trailer is behind. I do wonder if under a light load and a close eye on the temp gauge that maybe they did get away with it like that, as already mentioned. Then when Doug hooked the trl on and put it under load, yep you guys get it. Sounds like it could be a easy fix , especially if the head gaskets are okay, hopefully it didn't get too hot.
As far as the price of the bus goes doesn't look over priced to me , even if a repair is neccessary but then again we don't have such a nice crop to pick from up here.