Trailers that have sat outside for years or decades may need more than a simple clean--they may need a full restoration. First, inspect the trailer carefully, especially the axle and frame. If there is serious rust damage and structural issues, you might want to look into scrapping a camper rather than cleaning it. If the camper checks out structurally, undergoing a restoration might still be a bigger job than you want to partake in. Water leaks often mean you'll be replacing cabinets, walls, and under flooring due to mold, mildew, and rot. If the trailer isn't that bad, cleaning it shouldn't be a big deal. Steam clean all upholstery, and use degreasers and regular RV soap to wash the exterior. It may take a while and a few coats of paint, but most RVs can be cleaned up to look pretty good!