Consider using the following or an equivalent checklist. Ignore the parts for motor homes or fifth wheels. This on is for inspecting a new trailer before accepting it, but it can be a good reminder for inspecting a used one. This checklist caused me to refuse delivery of a brand new fifth wheel a few years ago. The dealer was stunned, said they had never had a trailer refused. I told them there are a lot of naive people out there, and once they fixed the list I would pay for it.
http://changingears.com/rv-checklist-inspection.shtml