There is a company in Canada that sells a hitch that attaches to the pin box so that 2 or 3 bicycles can be carried at the fifth wheel pin above the bed rails. I looked at this some time ago and found it to be too expensive (about $500 to carry $400 in bicycles). This was also at a time where I had huge concerns about adding more weight at the pin/rear axle of my TV.
Here's the website:
http://toppoprails.com/
On the web page, click on the 7000-5W or 7000-5W-XL links for the instructions for the brackets. This could give you some ideas of what you are looking for.