If you are low on transmission fluid every so often, it is leaking. And the smoke that you were seeing is because the fluid is dropping on the hot muffler or cad converter. You probably blew a rear seal on the transmission.
A Ford truck will keep running as long as it has some fluid, but once it gets to low it will not pull much then.
You claim that the coach had not been run for over ten years. Seals dry up and that is probably your problem with the rear or even a front seal. Rear are easy to change out, but the front you will have to pull the transmission.