Nancy & Les,
When cutting the PVC, make sure you leave enough pipe on each side of the cut to fit inside a coupling. Lowe's, Home Depot or any good plumbing supply should have the coupling, primer and adhesive to put it back together. Take with you, one side of the piece you have cut so you purchase the correct size coupling. It's no more difficult than repairing a PVC sprinkler system except you don't have to dig it up.
I'd suggest cleaning the pieces to be re-glued with fine steel wool or sand paper. Scrape off any burrs inside and outside the pipe, created with the cut, so the coupling will slip on without jamming and solids inside won't get caught when draining. If this is a completely new experience for you, pick someone's brain before getting started. A neighbor, plumbing salesman, relative or---?
Worst case scenario, if you should feel totally intimidated, tow the trailer to a plumber's and ask him to fix it. For one who has all the pieces, tools and experience, the job probably won't take 1/2 hr.
Enjoy your unique experience, not too many of us get to play with the big size plumbing under our trailers.
Kurt & Ann K.