How old is too old, subjective question but I want opinions

Personally, I wouldn't go much earlier than a 1995 rig. Prior to that, there were still a few carburators around and EFI was a new technology that had birthing pains. Our first rig was an '85 26' C, with a carburated Ford 460. What a turkey. It had enough smog gear for a small city and got 6 mpg on a good day, dropping to about 4,5 in the mountains.

Or current rig is a 32' Class A with the Ford V-10. 8 mpg consistently, regardless of elevation.