Two options... Do you have any kind of bumper at all back there? They make bumper mounted bike racks designed to mount on a square or "L-Bracket" bumper.
Page 2 is do you have a bumper on the FRONT of your tow vehicle... Same thing there, slightly different mounting bracked
Option 3 is to have a 2"receiver installed on the Trailer nad hang a bike rack on that.
Options abound. There are additional options that come to mind if you don't mind drilling into your rig, and if it can support the bikes (not all can and I('d not care to drill into it)