Welcome to the forum!
Frankly, the value of a 40-year old 5th wheel is pretty much what someone will pay for it.
RV's, unlike fine wine, definitely do not get better with age. Quite the opposite.
If you're interested in selling it, you might try listing it on RVTrader.com -- however when I just checked it there's not a single Collins product currently for sale nationwide.
Maybe some others will chime in as well.
. . . never confuse education with intelligence, nor motion with progress
