I don't use any lines or marks; I just go by how far the camper is off the drivers' box rail... that may or may not work for different camper, with mine I have tanks on that side and lots of room on the blind side so if I just watch as I go by the grey water tank and keep it about 1" off the inside of the box rail when I sit it down, I'm pretty much perfect.
I do use a line on my 5th wheel parking though, but that's just to position it so I have the proper space on both sides when it's parked for putting slides out and such.
Steve
2014 F350 6.7 Platinum
2016 Cougar 330RBK
1991 Slumberqueen WS100