Water damage in walls

If the friend is married, get his wife to look at it - that will be the end of the deal for sure.

For the past five years, the RV has just been driven around the campground. I don’t think the walls will fall apart while driving

Are there any soft spots in the floor ? Depending how level the rv was parked will determine how the water ran. Leaks don't always flow straight down. They can travel in any direction to find the lowest point.

I would be concerned about the tires. Anytime the salvage tow operators moved older rvs , the tires were the main problem.
Time and UV is the enemy causing sidewall cracks. Tires can also suffer internal damage over time where you can't see it. Many rv people change sneakers after 5 yrs or so, just to be safe. If your tires blow up at highway speed it won't matter much how soggy the sidewalls are.
I would also check the front end for worn parts. Check the frame as well for rust.

I am no expert on mold but I would be concerned if I had black mold developing behind a wall.

Due to various degrees of rot I really don't see how anyone would really know the answer regarding highway safety without inspecting the unit.

For the past five years, the RV has just been driven around the campground. I don’t think the walls will fall apart while driving.

Clicky: https://www.amazon.com/XFasten-8-Inch-30-Foot-Sealing-Patching/dp/B07663NFZZ/ref=sr_1_1_sspa?crid=KF8VDO7BT7WY&keywords=rv+butyl+tape&qid=1569672046&s=gateway&sprefix=rv+bu%2Caps%2C230&sr=8-1-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUE3UUFDNVQ0RzdFUUgmZW5jcnlwdGVkSWQ9QTAxMjQzNDIzTVMzOEw3UDhWRU1TJmVuY3J5cHRlZEFkSWQ9QTA2NTY1MTdKS0c4Mjc3VklYOUQmd2lkZ2V0TmFtZT1zcF9hdGYmYWN0aW9uPWNsaWNrUmVkaXJlY3QmZG9Ob3RMb2dDbGljaz10cnVl

Old motorhome have walls that fall apart thanks to the windows leaking between the frame and wall. With any RV, you need to go around and tighten the window frame screws once a year. After about 10 years the butyl tape used a sealer must be replaced as it will be dried out and fully compressed allowing the frame to contact the wall directly and it will leak. Sealer smeared around the perimeter of the window frame is useless. It will leak again very quickly. Do it right. As for the RV, $1700 is probably way too much for something that may turn out to be junk.

Clicky

How bad are the walls?
Has the RV been driven lately or has it been sitting for years? Will the walls self destruct while the RV is being driven.
A picture or link would be helpful.