what to look for on used purchase

Sounds like it is a good find because it has been stored inside and protected from the weather.

Start at the top and run a water test across the roof, make sure there are no obvious leaks.  Push on the walls (not too hard) and make sure there is minimal give.  Push on the floors, especially around the entry, looking for any excessive give as well.

Run water through it and look for leaks, as well as put some water in fresh tank and check the water pump.

Make sure all the systems work, refrigerator, stove, sink, and any extras.

Don't worry too much about the tires, just make sure they can get you home, if you purchase this unit you will be replacing the tires at that age anyway.

@aquamous, I don't know if you read this post by @FishDoctor, but their list might be a good one for you to use in checking out a used T@B too, not just a new T@B. At least you could glean some ideas!