I know the distance between my jack feet and my tires from when I first offloaded my TC (or any other time I've offloaded, it's always the same). Now, you can use lots of different things; tape, paint, rope, something long and straight and make a straight line out 10-20 ft out in front of the TC. I just use my drivers side mirror and put the outside of the tires on the line and slowly drive back. No neck twisting involved. Starting out 10-20' and knowing the correct lineup way in advance really helps.
Hop out and take a look. Seriously, when you're 1/3 of the way in, jump out, run back and see if it's lined up. Keep jumping in and out of the truck and checking your work. Eventually you will get it right on and you will develop an idea of what that looks like from the drivers seat.
THANKS FOR ALL YOUR REPLYS!. I was checking off and on all day yesterday and was pleasantly surprised with all of the help and ideas that were posted. Again, Thank you to all. One question though....how did that one guys get that camper in backwards??? I thought I was having issues :~)
I measured out from the center of my truck bed to the rails and the center of my TC to the overhangs. Placed a piece of tape. Back under and have the tape follow the bed rail of the truck. Focus on one side, monitor the other side - both in the mirror. Keeps it straight. Rear of TC (where it juts out) has about 3/4 inch space when reaches the rear tailgate area of the truck. I have it marked front, middle, and rear. For the middle of the TC wing, I measured off the outside of the TC for the front and rear.
*Reason for the wood along the base of the TC is that my TC is for a mini truck and I have 44" between the wells. A 1/4 inch wedge keeps it centered which I drop in as I lower it.
May give you another idea. Combine all..or some... to make it work for you.