Welcome George! First of all, the rotten floor issues are, in my opinion, quite uncommon. The one issue that I see quite often are roof problems, so make sure to check the inside ceiling for any ripples. The ripples indicate there's been water coming in at one time. If possible, I'd make sure to get up next to the roof (don't walk on it) and make sure it looks solid with no tears or significant wear. I would try to get as much background info on each coach as possible so you can get an idea on how well it was maintained, etc. The fewer the miles on the coach, the less likely you are to have problems. When the coach has seen more road miles, things shift which can lead to a leak if the roof isn't properly resealed within the recommended time period.
Whichever one you decide on, check every plumbing fitting you can see for leaks. This would be best done before you hold title to it, but that isn't always possible. Doing this can prevent most floor rot problems. Most problems originate near the fresh water tank or the water heater.
One thing I would make sure to do is test all appliances before you take it away. I know this isn't always possible, but try as hard as possible to get some power or water out there to try it. You wouldn't want to buy a coach and find out it needs a $1000 refer.
If any of these coaches are advertised online, please post the links. I can look at the pics and tell you if I see anything that looks alarming that you should watch out for.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR