I mounted Thule Racks to the top of my tool box behind the cab. You have to remove the front wheel and I do have a slider hitch which keeps the bikes from hitting the front of the Monty when turning. It works extremely well. I did have to do a little support strengthening for the tool box lid though. They are about $20 each. - here is the link. you can buy at any good bike shop.
http://westernbikeworks.com/productdetail.asp?p=TJLOW