The towing and payload capacity of a 2013 F150 FX4 will depended on how it is configured and equipped. I saw payload numbers from 1,570 lbs up to 2,810 lbs and maximum towing numbers from 7.500 lbs to 9,700 lbs depending on equipment and configuration. You need to KNOW what those numbers are before getting to far into looking for a trailer unless you're also considering getting a new tow vehicle.
If you are handy and willing to work on things electric, mechanical and plumbing then you might want to considered a used unit. A travel trailer is part house and part vehicle so if you're confident in your ability to inspect/find issues in these realms and willing to fix what you don't initially find then you can save a lot of money buying used.
If not, and you're going to buy new, then try to settle on a unit your tow vehicle can handle with a margin of comfort, check out dealers within a reasonable driving distance that have it and get as many reviews as you can about the dealer and their service as possible.
You're going to hear all kinds of stuff about the quality of this or that brand or model. Take those into consideration but in my opinion the dealer plays a huge role in the perception of quality, is an important link in the quality assurance process (catching issues before the customer every sees them) and will play a major role in whether or not you are satisfied with your purchase.
