You will of course have the option to buy new or used. While a used camper could end up saving you money initially, sometimes in the thousands of dollars, you should be aware of why the person is selling it. Often, they may be unwilling to execute a substantial repair, the price of which will eventually be passed on to you.
Your truck camper purchase will ultimately reflect what amenities you want to add. For example, if you consistently vacation at family-oriented campgrounds, they often have restrooms, so don't pay for your own bathroom unless you have to. If you go out in the cooler months, then you may want to spring for added insulation.
Some suggest that you choose a camper, then the truck to carry it. However, if you already have the truck, then the camper will need to conform to the truck's load-bearing capacity. Alternatively, if you purchase a new camper first, you can defray some of the costs by buying a used truck, an option that is often less dangerous than buying a used camper instead.
