Lance camper truck bed width opening question

1988 is the year most makers narrowed that clearance. That’s the year Chevy narrowed their tailgate opening. (Dodge did it in ‘94 and Ford in ‘98.)
The clearance issue is usually only on one side, where there’s a little bump out in the area inside the bed behind the wheel well, for the camper bathroom.
You can modify an earlier camper to fit. That bump-out just needs to be trimmed down two or three inches. Often there’s enough space to do that without much effort.
On my ‘87 Vacationeer, the fiberglass wet-bath shell was right up against the inside of that bump-out section. I sliced the fiberglass and sectioned a couple inches out, then ‘glassed it back together.

If you fund a killer deal on a well-preserved older camper you really want, the tailgate clearance issue isn’t a deal-breaker, and you can get it home in your newer truck, you’ll just have to load it a little crooked and look like a dummy on the way home lol.