Well, I confess that when I first read your post I thought you were just winding us up with a yarn about one of those infamous "barn finds." The pix give credibility to the story.
That's a very nice bike. Looks like there are some mods: aftermarket windscreen, paint, saddle. The center stand is knackered, so you may want to address that fairly early.
Do take the comments about the valves closing up seriously. I do not agree that it is a gradual process. One year I road across country on my RS of that vintage and the exhaust valves lost ALL measureable lash before I reached the rally. Ignored, that would have distroyed the top end, at the very least. That can be costly.
The RS is my favorite model Airhead. The fairing is a design marvel, giving the rider great protection without being a barn door on the front of the bike. If the bike suits you, I'm sure you'll want to upgrade the suspension and brakes. With careful mods, these old bikes are still excellent sport tourers.
Congratulations on a real bargain. You're the envy of many!