3 Horse,

The tow weight is different than the 5th wheel weight.  Towing an 8000 lb trailer puts +/- 800 lbs on your bumper hitch, and therefore onto your truck.  Towing an 8000 lb 5th wheel/gooseneck puts +/- 2000 lbs directly over your rear axle and onto your truck. 

There is a difference between towing capacities for bumper pull and 5th wheel/gooseneck trailers. 

here are several things to consider in what a truck will tow.  How much power the truck has, the ability to pass that power through the power train to the tires, the ability to stop the load when braking, Shocks, springs and other suspension componets.  These all are factored together to calculate the trailer weight you truck will tow. In your case 8000lbs. This is usually calculated by taking the GCVW ( the weight of both the truck and trailer) and subtracting the weight of the truck to leave the max towable weight.  Most truck manufactures advertise this number based on their lighter trucks, since they have a higher tow weight.  A regular cab, Short Bed,2 wheel drive truck weighs less than a Crew Cab, Long Bed 4x4. So be carefull which spec sheet you look at. Make sure it's the specs for your truck.

Then you need to look at where the weight of towing is carried. As stated a gooseneck of 8000lbs will put 20%-30% of it's weight on the truck (depending on trailer design)  25% of 8000lbs puts 2000lbs on your trucks rear axle. I think you said in a previous post your truck is rated somewhere in the 1500-1600lb range for  payload. So even though you are within the rating of what your truck is rated to tow (8000lbs),  you are over the payload limit of what weight you can sit on top of the rear axle (1500 lbs)

If you through a few bales of hay in the bed, have you two roping buddies jump up in the cab with you, The truck is over weight.