The things I would be looking for,
Canvas for any tears or mould
zips
fly screens
inside trailer for water leeks and general condition
good look at chassis, see if it is straight or any cracks or old cracks welded up
crawl under and just a general look, oil leeking from shocks, cracks, rust.
If it has a kitchen, make sure all works, gas etc.
Minor thinks can be fixed, just need to allow for in the cost, but I would stay away from anything that has poor canvas, twists or cracks in the chassis and any rust(chassis or body).
Hope this gives you a starting point.
Trevor