I have seen people use stick on magnets, and when you have it perfect, stick on a magnet on the front of the camper, and the back of the camper and have two more magnets that you stick on with wands out from there. You make a mark in your left hand rear view mirror with a felt pen when you have it perfect, set the wands to the perfect length and they should be the same front and back, and then line up the tips of the wands with the felt mark. If everything lines up, you will be in exactly the same place in your box.
