I custom built a rack on the back of my 30' 5th wheel for a 300 pound dirt bike. With the weight of the custom built rack (never weighed the metal) and the 300 pound bike, that added "WAY" too much weight to the rear of the triler. It whipped and swayed like a regular trailer would that was loaded with too much weight to the rear. The great thing about a 5th wheel is the way that it tows like it's not even there. You put a 500 lb rack and an 800 lb bike on the back of your 5th wheel, it might make it so hard to control that you will never use it. I would seriously consider a smaller trailer to pull behind the 5th wheel just for the Goldwing. It would make it much better handling.
I recently met a couple who are full-time rv'ing in a Montana Big Sky 5th Wheel. They have a Gold Wing motorcycle and carry it on the rear of their 5th wheel with a lift that was purchased and installed by Eagle Head Lifts. I asked about the weight and they said that some type of change needed made to the 5th wheel to adjust the center of gravity. That was also done by the people at Eagle Head. Maybe the web site will help you out (the bottom row of pictures are of my friends Michael & Layla! www.eagleheadlifts.com