With that amount of trailer weight I would be looking for a used 3/4 ton pickup with a gas engine and 4.10 gears. I would try to get at least a 5-speed transmission and want a truck that had the factory tow package with its tranmsission cooler and often extra radiator core.
Big difference between the maximum load a truck can tow and what is designed to tow on a regular basis. Big difference in the braking capabilities of different trucks and one range of models may have marginal brakes and at a redesign or new platform introduction the brakes and suspension and other point of failure in the past have been redesigned to hold up better.
People tend to look at only the engine but the cost to repair an AC can be $1100 or more. Cost to rebuild the transmission can be over $3,000. Different makes and model years had varying degrees of problems with the cooling system, electrics, suspension, brakes, and other parts of the truck. Consumer Reports has a guide based on the survey they do of all their members each year where people provide information on the vehicle, model year, and any repairs that were needed. With tens of thousands of trucks it is easy to spot trends where one truck is more reliable than others.
For my money for towing a trailer my first choice would be a Toyota as they have the best track record for reliability overall for each of the last 5 years according to JD Powers and the 5 Toyota trucks I have owned were far more reliable than the 8 trucks I have owned from Dodge, Willys, Ford, and Chevrolet.