I tow at whatever is a safe speed for the road conditions and traffic conditions. (also taking into account the max speed rating of 75 MPH on the trailer tires)
Usually it ends up being between 65-68.
I do get significantly better MPG at 55-57, but over 7 days driving, that extra 10 mph slower would make it 8 days of driving instead of 7.
RE: the OP concerns of "getting run over"... never had anything like that happen/or come close to happening. Even when in SD or Montana where most everyone is running 85-95. They just go around me, never had anyone run me over. :)
If they're jammed up because of a slow moving semi, if it is safe I will speed up a bit to lower their blood pressure. But they couldn't "run me over" even if they decided to try.
When not towing the trailer I ride a motorcycle as my daily driver. I do what is safe for me...on the bike and in the truck. No small part of that is staying away from the herds/clumps of vehicles all traveling fast and too close to each other.
I ESPECIALLY put a lot of distance between me and a truck pulling a trailer at 75-85, wagging their tail down the road.