Bought a fifth wheel that has sat most of its life

The outer face of the tread is not as important as the condition of the tire between the tread. However, a tire sitting on the ground for 2-3 years, without moving, can be an issue. Tire quality can also make a difference, as there are many cheap trailer tires, that don't last long, even when they are used on a regular basis. I'd watch them closely, if just using for shorter trips. If heading out on a cross country trip, I would consider replacement.

Jerry