This is the intended behavior, however, it seems the mission is currently glitched/broke and tells you to tow the tow trucks. It's been reported already and being worked on.
In the mean time just avoid doing it.
You should be ale to tow other cars still, however. Pressing the 2 key to find a vehicle to tow only looks for the closest one, so just drive away from gas station and try it and it should find a different vehicle.