You can go to any of the big-box stores and get a "Shark-bite" fitting that will attach to the cut pipe and have the female threaded end to attach to the toilet's male threads.
You can also install a valve into the supply run that will permit you to turn off the water to the toilet. That may be something you wish to have should you have a problem with the toilet and need to work on it or replace it. With a valve, you don't need to shut down the water supply to the rest of the trailer.
If you plan to replace more of your plumbing fixtures or anticipate modifying other supply lines, you can forgo the easy Shark-bite fitting and buy the necessary crimping tool and crimp rings to attach all manner of plumbing fittings. For a one-off job, the Sharkbite fittings are hard to beat.