In it's day that was a good truck for towing. It can handle the weight for sure, just light on HP.
I'm not sure how you're relating truck wheelbase to camper length but I think a 3500 long bed should be fine with a longer trailer. The overall problem is just that you will be quite slow going up hill. Expect to have your foot flat on the floor in second gear; or maybe even first gear if it's an automatic.
How much did the GN horse trailer weight? You should already have an idea what towing with this truck is like. An RV should give a similar experience but it will be less aerodynamic.