A F-250 or F-350 are the most likely trucks that you'll come across. In the 1970’s Ford had a heavy duty F-150 that would handle this job. Basically, it was a 3/4 ton truck equipped with ligher springs and 15 inch tires. To handle heavy loads, it had factory overload leaf springs that hit their pads after an inch or so of rear end squat. My 1978 F-150 was equipped from the factory for heavy towing and was used quite adequately for a 10,000lb 5th wheel trailer. If you do come across one of these beauties, the tires are likely to be incompatible for a heavy tow as the 15 inch wheels made it too easy to save some $$$ and put some non-LT rated tires on it.
Most of the talk has been on engine. Yeah, get a diesel or 460 V8. That will get you down the road without blowing the engine but other items will keep you from wrecking. Tighten up the steering via tie rod ends so the truck doesn't wonder around and will handle cross winds better. Check out the bushings in the rear leaf springs for the same reason. A heavy tow really pushes the back of the truck around and the rear suspension has to be tight. Put on new shocks front/rear (Rancho on a budget and Bilstein for high end). A transmission oil cooler is mandatory - Aftermarket coolers are a plus even if the truck has an OEM transmission cooler integral with the radiator. Flush the cooling system then pressure test. The pressure test will probably show up a leak if the radiator is weak from corrosion. Complete fluid change out for engine, transmission, differentials, power steering and brakes. Inspect the brakes - Besides the pads, wheel cylinders and such, the master cylinder could be weak. Look at the wiring to the rear lights - Repairs could be needed to fix sloppy splices made over the years for trailer light harnesses. Obviously, an electric brake controller is needed although you could find a truck in this category already has one. An auxiliary tank is a plus - If the wind picks up and in the mountains, the MPG can be cut by half.
Be cautious about trucks that are anything other than OEM suspensions. There are lots of sloppy suspension modified trucks with lousy suspension mods and body lifts around so they could fit some oversized tires and look cool. No body lifts period, no blocks on the rear leafs!
Just realized that I am rattling on now. Better stop! LOL!
Thanks. Sounds like good advice (Mainly because it pretty much mirrors my own thinking)