I think garyemunson nailed it; this would not be cost effective to repair if you hire it out, but may be if you do it yourself and your labor is "free." Even then, it's going to be a significant project to undertake.
Expect the water damage to be more widespread than you initially see; I wouldn't be surprised if a portion of the floor is also at least partly rotted, and maybe more as well. If the trailer framework is wood, that's probably compromised in the affected area, and should be replaced for structural soundness. If it's an aluminum frame that obviously is not so much of a problem--aluminum doesn't rot.