Well, since you don't know about any leaks since its a new to you trailer, I would not suggest a high powered car wash or power washer. Wash it at home with a long handled brush made specially for RV's where the water goes through the handle. Use a mild detergent along with the brush. What kind of interior water damage do you see on the cabinets and floor? The windows and vents will be your biggest leaking culprits so look below them. Seams can and will leak too, so look everywhere for any evidence of prior leaking. According to the VAP Panel Pros, "they all leak."
Good luck and post some photos when you can.
4CU 2699 / AIR 10 / TAC AZ-1
I'm haunted by aluminum.
Charter Member of the 4 Corners Unit.