Back into your question by determining how much your max cargo load will be. Then add weight of truck and trailer. This will give you GCWR. Research the manufacturers Gross Combined Weight Rating for the truck, trailer and load of the truck you want.
Example: My Dodge 2500 has a GCWR of 21k. Truck weighs 7k. That leaves me 14 k for trailer and cargo. My initial thought was bigger is better so looked at a 22k dual tandem GN trailer that weighed 7k empty. Leaving me 7k for my cargo. That was borderline for what I wanted to do. I ended up with a 14k single tandem that weighed 5k. That left me with 9k potential cargo. More than enough to haul my Farmall SH's & SC's.
DOT is looking at GCWR in TX. I've also had inspector look at my tires to insure they where rated for the truck.
Others claim DOT is not enforcing that and go by registered weight. I suspect that once all the various Law Enforcement Agencies figure out they can make fine money they'll all enforce it.