It would help if you described "sitting on a gooseneck type adapter" in more detail. Pictures would help also. It could be one of several things.

 

Q? Is your truck pre-provisioned for either/or type hitches, or will you need to install whichever you go with?

Pros: One big advantage of a goosneck is a weight reduction. A typical fifth hitch weighs 200+ pounds - that counts against your available payload. It can also hitch/ unhitch at a wider angle.
Cons: its not as massive (implying strength); you know, more is better?

I went with the Anderson Ultimate Hitch, about 40 pounds and can easily be removed to free up the bed. The red part remains on the 5th, the rest goes in the bed of the truck.

 

Check with the trailer manufacturer to make sure the trailer is rated for a gooseneck type hitch.

 

 

 

There are 4 big advantages:
1) The bed of the truck is available for hauling large objects without removing the kingpin hitch.
2) Smoother driver experience due to reduced chucking.
3) More of your payload capacity available due to not having said hitch in the bed of the truck.
4) Hitching and unhitching at odd angles that a kingpin type hitch will not like.

It is unclear from the OP's text how the trailer is equipped. There are three scenarios I can see from his description:

1) Kingpin type pin box with a gooseneck adapter, or
2) Reese Goosebox, or
3) Custom Gooseneck attached to the trailer

I would steer clear of #1, due to the adapter voiding the frame warranty if it is a Lippert frame. #2 is a better solution, as the Reese Goosebox is acceptable to Lippert. For disclosure purposes, I tow my 40' Cardinal with the 20k Goosebox. As for #3, if the custom gooseneck was smartly designed and PROPERLY welded AND gusseted into the frame, it should be fine.

As for the truck, if it has the hitch prep, all that is needed is the ball and the tow chain attachments. My experience has been that only California and West Virginia require chains for the gooseneck. Your mileage and experience may vary.

If it does not have the hitch prep, there are many options for adding an under-bed gooseneck ball mount. My personal favorites are the B&W Turnover Ball and the Blue Ox Gooseneck hitch. I have installed both, and the both work great. I lean slightly to the Blue Ox, as it uses a locking collar in the bed to hold the ball in place, instead of the lever and pin under the wheel well. (Actually, DW prefers the Blue Ox because it keeps my shirt and jacket sleeves cleaner... Happy wife, happy life!).

There are also gooseneck adapters that will mount to the ubiquitous in-bed rails, made by many manufacturers.

I think that covers it. Did I miss anything?

 

I second the Anderson Ultimate Hitch. Pulls our 38' toy hauler just fine (no clanging or bucking even on extreme bridge transitions) attaches to the factory ball and is extremely easy to remove and install.

 

Lippert states only the Reese GooseBox will not void their warranty on the frame. Added to our Montanan HC and pulls well. Has an air bag to dampen bumps. Lining up needs to be precise. Another option would be the B&W Companion which breaks apart into 2 parts of ~75# each to remove. Installs into the pucks in the bed of you have the factory towing prep.