Depending on when in the 80’s it was built, yes they just bolted through the floor. Mine was that way originally. When I bought it it didn’t have any floor covering and the holes were there. Someone along the way added four eye bolts on the corners.
What to look for? A lot of good advice thus far. Especially the sit in it awhile part. I own an 80’s vintage, here’s my take:
Most of that vintage had the insulated soft side. They had a propensity to get moldy and can’t be easily cleaned. Most people replace them at some point.
Look closely at the roof where the side trim meets for good seal. On the early years the soft side was attached outside the roof panels and covered with trim. A leak often developed and soaked the soft side. Later years the roof panels were folded over the soft side. A lot of people pull the trim and bend the roof panels over the outside of the soft side.
The Roof is two piece with a seam down the middle and screwed to the frame. Look that the seam looks well sealed and the screws are well covered with dicor. If the seam looks iffy look closely at the roof inside for water stains. A lot of people run a wide strip of Eternabond down the seam to seal it. Lots of large hail pockets un-filled need to be examined. Somewhere on here is a post of an old Grandby rebuild. When they pulled the panels to re-insulate the roof, several pock marks had penetrated the roof.
If it has the louvered windows, check that the cranks work correctly and easily. The crank assemblies can be hard to find at times.
The advice to look for cracks or buckling in the walls is sound advice. To get the “flex frames” they advertised then, the wall studs weren’t welded all the way around. Several of the rebuild threads on here over the years mention finding broken frame studs.
Look close at the water tank. Some tanks used had a center seam that had a propensity to develop a leak. Look to see if it’s flaking or wiggly at the seam and for water staining on the floor. The old units tended to ride heavy to the street side. Some people move the tank to the front center compartment and install a better electric pump to move water.
The electric system, at least on mine was dirt simple originally. It was 12 volt only with a 4 round automotive fuse block. The power to all the overhead and outside lights runs to roof behind the front panel. Look as best you can that the harness is still supple and not getting kinked by the panel folding on it.
If all looks good, smells good, not suffering from overly poor craftsmanship and as measured will fit your truck, don’t fear an older one. There isn’t that much to one, for the most part repairable, and parts are available. I had a camping buddy who owned a Keystone for near 40 years. It was still fully functioning at the time of his passing.
Good luck with your decision.