Do yourself a huge favor and thoroughly test drive it. I'm talking at least and hour, preferably 2, on as many different roads as you can {ideally on a very windy day}. Get out on the interstate and see what it is like to have an 18 wheeler blow by you at 80.
Find a 6 per cent or more grade and put your foot to the floor. Can you live with the engine noise at 4,000+ rpm {some can-some cannot}. Most owners have had their rigs weighed at a CAT Scale and should have the weight ticket for you to review. Check the tire pressures based upon the scale weight and the tire manufacturers inflation table {easily found on the internet}.
Does it have the 55 gallon tank or the 40... most folks find the 40 is not enough. As noted check the DOT codes to determine the age of the tires {does it have a spare?}, even if they look good any over 4 years old are suspect. With 55K miles however it may very well have newer tires but keep in mind that if needed a new set is between $1,200 - $1,500.
Thor is an entry level unit so inspect thoroughly with that in mind. Look for any signs of water intrusion/leaks and if you find any... Run Forrest Run! It probably has the Ford V-10 with the 5 speed torque Shift transmission, both are nearly bulletproof and hopefully the seller has complete service records. Expect 9 mpg and be happy when you occasionally beat that number.
These are just a few things to check BEFORE you sign anything much less hand over any money. if the seller balks See RFR above. If you know anyone with RV experience take them along and then buy them lunch afterwards where you can discuss what you found. Good luck!
:C