Welcome! Sunline did make a number of 5th wheels, but they were not as popular as the travel trailers . And we do not have that many 5th wheel members here in our club so that can answer why you cannot find much on them.
But, don't let that stop you. We know the Sunlines and are here to help as we can.
First is some info on the camper. I'm not sure if you found our FILES section yet with the Brochures section. We have many original sales brochures on the Sunlines and your 1988 camper is in that section. There is a lot of good spec info in there.
To get to the files section, you need to be logged into the forum.
At the top of the forum are tabs, look for the word "FILES" click it.
Once into the files section, scroll down to the "Sunline Brochures" section. Go in there and find a sale brochure for the camper you are looking at and down load a copy. Empty camper tongue weights/pin weights are shown there on most models. But remember, the tongue/pin weight goes up from the brochure dry weight when you load the camper.
Your F250 should handle that smaller camper well. The 88 F2050 was listed as a 4,480# GVWR camper when fully loaded with a 600# empty camper pin weight.
One heads up when buying a used camper of any kind or brand, tires and water damage. You can check the date codes on the side wall of the tires, and if they are older then 5 years, or worse have no code at all, check the side walls inside and out, trailer tires start breaking down inside from age regardless of the thread thickness. Not sure how far you have to tow it to get home, but air them up to max side wall pressure and take it easy as when an old tire heats up, they do come apart.
Water damage. Campers leak different then a house that you can see a leak more easily. Many times you cannot see water damage buried in a wall, ceiling or floor. If do see water stains in the living space, or smell musty mildew, then the leak could have been going on a long time even if the wall feels dry.
There are ways to inspect for water damage, the better way is to use a moisture meter. You can scan the walls etc and see behind them if water damage is in the walls, ceiling and floor. See this post for more info. I recommend you get a meter and check the camper before you buy it so you know going in, eyes wide open what to expect. The prior owner may not realize what is going in in their camper.
Also, the moisture meter will not read frozen wood correctly as we get into the winter months. It will read 0% when trying to measure snow or ice. But it can read wet water perfectly.
Not saying to not buy a camper with some level of water damage, just pointing out to check it and not pay a large price for what you thought was a dry camper as you cannot see any wet spots. Any of the Sunline are repairable from water damage, but it normally only makes financial sense that you can do the work yourself.
Hope this helps and good luck. We are here to help, just ask.