The three things to look for in a used trailer are leaks, leaks, leaks. Yes I this know from experience. Get a good BRIGHT flashlight and look into every place you can, especially cabinets under tub/shower, storage compartments. Walk the roof looking for soft spots, and signs of damage. Also condition of roof. Is it clean or very dirty, which shows lack of care. Check all vents and roof penetration's. Look around all parts of slides in and out, for signs of water, either damp or stains. Check every appliance to make sure that they work. Pull out awning. Look under trailer for any signs of damage or again water damage. If trailer is open below, feel along inside of bottom of walls for soft spots. especially at corners. Check all corners inside and out.
Also what kind of weather will you be living in? If is will be very cold out, most trailers will be uncomfortable unless they are made for extreme weather.