http://www.safarimotorcoaches.com/service/addendums/2002/rm_chassis_hitch.pdf
maybe this people can helped you out??
mostly the rule is: GVW 16.000 lbs, + trailer (own weight,lets say +/- 4.000 lbs) you will have a payload of 12.000 lbs, depending on axle-tires-springs from the truck/trailer (5th-wheel) combo