Not a target on the back wall but reflective (3-m) strips you can buy at most truck stops.
First, park the thing, have a partner guide you in so you are as close to perfect as possible.
NOW get a laser level, or a laser pointer (level works best) and using masking tape (So it's easy to remove without damage) Tape it to first one side and then the other of the RV pointed at the wall where you will mount the strips.
Put strips where the red dot is
That easy.. Back in so you can see BOTH strips.
Also, you need one Ronan and Kunzel TRIP STRIP (or see last paragraph)
Once in the garage,, Mark where the rear tires are.. Then dig a small "Ditch" the width of the trip strip. insert strip and cover with concrete patch, Hoook strip to light.. When light comes on,, APPLY PARKING SYSTEM, you are there.
(R&K is a multi million dollar research and developement company in Marshall Michigan, one of their products is automatic doors, the "Trip Strip" is an old fashion automatic door mat about 1" wide designed to be placed UNDER concrete and sense the weight of a vehicle atop it. DING DING.. You can also use a gas station DINGER however on those the hose can move.. So the trip strip (Fixed) is far more accurate).
Alternative to the trip strip is a light beam door bell system.. Radio Shack used to sell these and all security stores do now. Set it so the RV breaks the beam and "Rings the chime" when it's backed in far enough DING DONG = STOP.