Our 1999 Tradewinds sat for about four years right before we bought it and I found no real ill effects. This was in New Mexico where it's very dry and for the most part critters aren't a major problem. You didn't say which model National you were looking at, but they were built well and hold up for many years with proper maintenance. You should be able to get the seller to forward copies of the maintenance records.
It's a given that after seven years you will need to replace the tires, so factor that in your price. Even if they look good they will be past the age to replace.
Here is a paper I wrote for a guy a while back. Some of it is about Tradewinds but a lot of it is generic.
I'd have a pro check it out for you. That would cost less than going there and finding out there are deal busters.