Underbed mounts are available for both GN and fifth wheel hitches. Weight difference between the two choices is nil relative to the total truck/fiver weights. My fifth wheel hitch breaks down quickly and easily when I need to carry a big load or to move a GN trailer around. I prefer being able to see the hitch from the driver's seat, makes hookup an ez single person hookup w/o getting in and out of the cab multiple times. Aligning an out of sight ball to the trailer is not near as ez and safety chains are required. Finally, should the increased stress cause frame damage, repairs are very expensive, not worth the risk.
A good alternative is the Andersen Ultimate fifth wheel hitch, which is really an elevated ball mount with a compact king pin to ball convertor. So this is a light weight ball hitch with the hitch point visible from the driver's seat and has minimal increase in stress to the fiver frame.
BTW, I had a 2001 Trail-lite, maybe the same model number, I can't remember. Anyway, the frame is definitely steel. The superstructure uses aluminum studs and the underbelly panels are aluminum. But if you look inside the front storage compartment or where the torsion axles bolt to the frame, you will see the steel frame.