This happened to me a lot when I had the trailer. It was easy to see behind the SUV, but people still kept trying to merge in between the SUV and TT.
I don't think it's about visibility - it's about people not thinking that those two vehicles are connected in the middle. They just see two vehicles and figure they can cram their way between them.
Not much you can do except to use standard procedures:
1. try not to travel in heavy rush hour traffic;
2. keep to a lane that has as minimal merging as possible (e.g., middle lane of three lane freeway);
3. keep plenty of distance between you and the vehicle in front of you - the 3-second rule - regardless of how heavy the traffic is. The more room you keep between you and the vehicle in front of you, the more maneuverability you have to avoid the idiot trying to merge between your RV and Toad. Don't worry that people will move into that empty space, just adjust and recreate the space - it really doesn't slow you down at all because you should rarely ever have to hit the brakes if you have that space between you and the vehicle in front of you.
4. Be aware of the drivers behind you and watch for signs that someone is likely to merge quickly (e.g., have they been changing lanes; is there a slowdown in their lane up ahead; etc.).