Payload is the bigger issue most of the time on everything up to and including 1/2 ton trucks.
For example. My 1/2 ton truck is rated to tow over 10k pounds however in the real world I’m going to run out of payload before I get to that point. The payload capacity on my truck is just under 1900 pounds (which is on the high side for a non-HD payload, non-max-tow truck). Let’s say I buy a travel trailer that has a towing weight of 9000 pounds, sounds good on paper right? Well below the max rated towing capacity. But in reality, it’s too heavy; here’s why
You want between 10% and 15% of the trailer’s weight on the tongue. We’ll split that, and say 1125 pounds. But we’re going to need a weight distributing hitch for something that heavy, so add another 125 pounds for that. We need propane for our trailer, and they are going to be located near the tongue, two 33 pound tanks will weigh about100 pounds, and we’re going to need some batteries so another 100 pounds there. We’re currently at about 1500 pounds of payload, and nobody has even gotten in the truck yet. So I climb aboard (we’ll call it 200 pounds), and now with the driver aboard we have less than 200 pounds of payload left for luggage/supplies and other people/pets. You can kinda see where this is going. Realistically a 9,000 pound trailer is too heavy for a truck that’s rated to tow 10,500 pounds.
On 3/4 ton and 1 ton trucks, with significantly higher payload capacities this sort of thing is less common, but on 1/2 tons and everything under that, the stated towing capacity is only happening if you have the unicorn half-ton ( Typically a single cab, long bed, lowest trim level, with the top engine, and the HD payload package and the max tow package (which isn’t the regular factory tow package).