I would even not be too concerned about tires and bearings - in the worst case, you can replace the entire axle, both wheels, and both tires for a modest fraction of the cost of the trailer. It is a good point, though, that if those things are well maintained, perhaps the whole trailer is. As with any trailer, water damage (perhaps due to plumbing issue, leaks at windows, or unrepaired collision damage) to the floor and interior are more important to watch for.
Other things - which again can go wrong in any RV - are fixable but can be expensive, so it worthwhile to make sure that major appliances (refrigerator, furnace, water heater...) work. The seller saying "I'm sure it works but it's not turned on because we never use it" is a red flag.