Your rear axle is most likely a Sterling 10.5 and the rating from the manufacturer is like 10K or close to it if memory serves me,so axle weight won't really effect your decision..It is way more than Fords stickers..
The Adventurer 89rbs is a fairly light weight slide camper if a slide is a must..Your truck will handle a fairly heavy camper but how comfy you will be driving it is another story...There top heavy and tippy in corners and make some people get rid of them fast because of the cornering....
A retired RV repair friend of mine bought a Eagle Cap 950 slide and put it on his newer SRW Dodge 3500 and took it home up a windy river road to his place near Elk City and sold it within a week because of the sway on the narrow road home...Said it needs a DRW because of instability in corners..
Good luck on your choice..