I use a north shore rack behind my truck and use a hitch riser from curt designed for a hitch mounted rack. They make a two, four or six and takes up the least amount of length room. Like said above, if you put too much weight on the back of the trail it can cause issues. When running bikes on the roof of the trailer, it puts a lot of lateral stress on the racks if off road.
CURT Manufacturing 45792 Dual 2" Receiver Extender in Trailer Hitches.
www.amazon.ca
If you look on my build I originally had the rack on the back of the trailer and found the bikes got thrown around. Newer pictures show it behind the truck.