To tow a travel trailer safely with your tire size and gear combination (this is just from my experience), you will want a trailer that weighs around 4,500# "dry weight", when you actually leave the dealership a 4,500# trailer will weigh much closer to 5,000#. Fill it up with food, gear, fluids, etc and you'll be pushing 6,500-7,000#. Most decent sized travel trailers have 80-100 gallon fresh water tanks, which will add almost 1000# on their own. Any higher weight than that will put lots of stress on your transmission. The engine can handle it fine, but for more weight than that (plus all the extra rotating mass from large tires) your transmission needs the leverage from a lower gear. Gears cost about $350 per axle minimum, and the labor is usually around $300 per axle.
Here's a pic of my setup to get you an idea of what I'm towing on a regular basis.
Doesn't look that big but it's a beefy frame, it's cargo capacity weight rating is over 4,000#