Tried this and it works great

I have a Dodge Quad Cab, so when backing up to the GN trailer I can't see the ball in the bed of the truck. This makes it a little hard to hitch up with just one person. The truck has a folding Tonneau so when backing to the trailer I line the tailgate lock (in the center of the tonneau) with the GN and have had fair success. Still have to get in and out of the truck a few times.

Today when I hooked up I took a floor magnet (the type you get at Home Depot for picking up nails and screws) It has about a 10 or 12 inch magnet and an adjustable handle. I put the magnet just behind the ball on the truck bed, handle vertical (plumb) off the truck bed. Now when I back to the trailer, I line the tailgate lock on the tonneau with the magnet handle and the trailer GN (center). I use the rearview mirror like a gun sight. When the GN bumps the magnet handle it's over the ball. Backed into it about four times today first time every time I was over the ball.

I think you could put a small decal low in the center of the rear window that would work the same as my tonneau lock.

Cost about $20 at Home Depot.

I use a scrap water copper pipe, PVC connector ring and duct tape.  I taped the pipe (about 3 ft long) to the outside of the PVC connector ring (6 in diameter).  The ring makes a good base and is heavy enough to hold up the pipe upright as well as wide enough to straddle the ribs in the bed of the truck.  Then I place the ring behind the ball in the truck.  When the screws of the GN hitch touch the pipe, I'm dead center over the ball.  I also use the ribs of the truck bed to line up the truck while backing.  I've got a Ford crew cab and can't see the ball from the driver's seat.  You can make a similar 'contraption' using spare parts around the house - block of wood, wooden dowel, old broom stick, etc.  I've seen similar items in catalogs go for $15 - $30.

BTW, best lesson I ever got in backing a GN trailer - line up the wheels in your side view mirror with where you want to go. Back slowly. Place hand at bottom of steering wheel. Turn wheel 1/2 revolution in direction you want rear of trailer to move (clockwise trailer end goes left, counter-clockwise trailer end goes right). WAIT for trailer to move. Line up truck with trailer once you are headed to target. Adjust as neccessary.  If you turn wheel more than 1/2 revolution at a time, then you are generally oversteering and the trailer will go all over the place on you.  One last tip - use all the area you can when turning around.

Yep, that works for me too. 

Exactly what I did, but I have a friend who cannot get her mind around this concept.  I have a tool box and I can judge the distance from it to the coupler to tell if I am right over the ball.  Never have a problem.

Yep, thats about the same as the floor magnet idea. It's real nice not to have to look back and strain your neck. Just aim like a gun sight, and drop the trailer down

My wife hooked up Sat. and again on Tuesday. First time and she was over the ball without help. She didn't have to look over her sholder Just line up in the rear view mirror.

As for using the ribs in the bed or tail gate. In our quad cab I can't even see the tail gate, so that isn't an option. Guess if I wasn't so short and old, or didn't have the quad cab with a folding tonneau I wouldn't need to find other options.

I use a tall toilet plunger placed just in front of the ball. Mark the plunger tip and the trailer (while hitched) with bright collored electrical tape. I use my inside mirror to watch while I back on the ball. When the plunger tip hits the spot marked on the trailer I am over the ball.

Just watch the smiles on peoples faces when you whip the plunger away. Great conversation starter too/