Do a thorough walkthrough and check everything. Make sure everything works - try it to be sure it works properly. Look in all the cabinets, storage areas for any damage, anything broken or evidence of leaks. Check for any sagging or bulging in the roof/walls, etc.
Here's a link to a prepurchase checklist that should help:
Also ask for the maintenance records & any the manuals for the stove, fridge, awning, roof, etc. You can almost always get the manuals on line if you have to, but it's better/easier if the seller has them.
Are the tires the original ones? If so at 6 years old they are getting a bit long in the tooth and you will probably need to think about replacing them soon. Could be a negotiating point if they are.