With any RV that sits for that long, check that the slide mechanism works properly, doublecheck the suspension and be very careful with the tires. Chances are, if the RV has sat in one spot that long, the tires will probably "look new" with adequate tread. Check the manufacture date on the sidewall. Most recommendations are to replace tires that are over 5 years old. They'll probably look new, but certainly won't "act new" once you get rolling down the highway.
There is a PDI checklist posted on the forum, It might be a good idea to print it off and use as a checklist to go over the "new to you" trailer so you won't forget to check something that may later be a "wish I had"
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS