Loading weight & distribution on a 5th wheel

As with a travel trailer. moving the center of gravity forward will make it more stable (same goes for all vehicles, including aircraft up to a point) but you don't want to overload your truck. You could weigh the trailer, find its center of gravity, then calculate the moment for everything you add to calculate a new center of gravity as with an aircraft, but what a lot of work. It ain't that critical. If in doubt, put it over or forward of the axles.

You will find your fifth wheel more stable when towing than your travel trailer so enjoy.