I took the easy way out and used a through the glass mount, as high as I could on the center of the rear window. The radio was around $100 and the antenna not very expensive, so I really did not have too great of expectations.
With the gooseneck trailer attached, rearward reception is extremely poor. My GN is 8 feet tall (bed to top) and there is an A/C unit on top of that. Everything to the rear is totally blocked.
I monitor conversations up ahead easily, but as someone goes by in the oncoming lane, the reception goes to zero as I lose sight of them over my sholder. I'm sure that is why the truckers mount their antennas as far out as they can on the mirrors, and on both sides.
I'm usually more concerned about what is up ahead so it does not bother me unless there is an interesting conversation going on and I lose it when I pass or they pass on the other side of the road.
I don't see a good way to mount antennas on the towing mirrors and I guess that an antenna on the trailer would be the best way to avoid the problem.