A thousand pounds was not too much for your transmission as long as the fluid was routinely changed, the cooling system was up to snuff, and you were good about not forcing it to change gears all the time. For example, I pull huge horse trailers with automatics instead of manuals these days. I turn off the over drive.

Yes, it's better to have an extra cooler, but staying a slower speed in a one gear should have been fine at that weight. Sounds like your tranny fluid was overheating.

I would flush the whole transmission, and give a check to make sure that the radiator and so forth is up to snuff.