In those days Camper Specials were designed to take 11.5 foot campers, but without slides and basements like they have now. Likely a heavy one would run 3500 pounds, and loaded up with stuff over 4000. Tanks were a lot smaller without basements so carrying 1000 pounds of water/black/grey just wasn't possible.
But towing.. I think you would be limited by the transmission and your desire to make it to the next gas station for the most part. 12,000 sounds very generous with the limits of slowing the beast downhill and braking.
