I would also recommend a thorough look over by an rv tech as well. However, if by chance you can not find one some questions to ask, or things to investigate on your own are:
1) Is it or has been permanently set up at this seasonal campground? Or where has it been stored for winter?
2) About how many miles has it seen towing?
3) Are you aware of any water intrusion issues (you can look for obvious interior staining around the slide outs, roof penetrations, windows)? Also check in basement storage area for any plumbing leaks or staining.
4) Are you the original owner? If so, can you tell me the service history? (look for brake and wheel bearing services, or ask what intervals these have been completed at).
5) not sure what your intended long term use for it is, but you may wish to know further details of construction, insulation, ect. if you plan on using it for extended cold weather.
6) Have the appliances been serviced? Confirm they are all in working order.
7) Confirm condition of roofing material and that all penetrations have been maintained / resealed as per manufacturer specs.
8) Confirm landing gear and stabilizer jacks are in working order.
9) Ask or look for any evidence of a rodent issue (poop in the cabinets or under drawers ect.)
10) Any accident claims history?
11) Fill all holding tanks and check for leaks and then good drainage when valves opened.
12) 2005, FYI your propane tanks should be coming up to date very soon, and will need to be replaced or recertified.
13) Tires were mentioned in another post. If original they will need to be replaced. Look for an significant cuts / abrasions on sidewalls. Check the condition of the rubber, is it dry and cracking?
That's the major items I can think of for now. Good luck on your purchase.