We have a Prowler Lite fifth wheel (825 5x) which weights about 4,500 pounds empty and probably close to 6,000 pounds loaded & ready to go. It is a 25 foot model but is 28'10" in total length front to back. If you assume 20% of that being the king pin weight, that gives you about 1,200 pounds of king pun weight plus hitch (200 pounds?) plus gas and passengers for your tow vehicle to be able to carry (plus pulling the RV).
Ours has bunks so would not suit you but it gives you an idea of a light weight 5th model & size.
Good luck, Paul