Towing tire pressures on short wheel base vs long

It's not about wheelbase, but about the type of tire. Your 1500 had P Metric tires and your 3500 has LT tires. LT tires support a quite a bit of weight at higher pressures. However, you can lower the pressure when you're not so heavily loaded. This helps make the ride a little more comfortable.

Each manufacturer produces something called an "inflation chart" for LT tires. It will tell you the minimum PSI needed in order to support different amounts of weight. Get a hold of this chart from your tire manufacturer and weight your truck with the trailer attached at a CAT scale.

Once you know how much weight your tires need to support then you can figure out how low you can lower your PSI for comfort. Doing it this way sure beats guessing or riding around at 80 PSI constantly.