I have planned to make a road trip to USA. I travel in a car with license plates and Mexican license but I would like to buy a pop-up trailer and be able to use it since I would be visiting several national parks for approximately 3 months. I would like to know if I can register and use a trailer and tow with my car with Mexican license plates
The catch is this: in what state will you register it? You either need to be a resident of the state, or need to qualify to register it there (which usually means you only use it there) . You'll have to check the main state of your operation to see if they would allow you to register a trailer there as a non-resident, and you'll need US auto insurance to cover it. It might make more sense to bring one from Mexico.
So the answer is this: maybe ... you'll need to contact states individually to see if you qualify for registration and insurance.
Incidentally, if a person considered doing something in Canada, the answer is the same and varies by province and territory in the same way. Just insert "province" or "territory", as appropriate, for "state".