One method is to place a short piece of black (or red, or some bright
color) tape in the center of the front of the trailer that you can see
from the inside rear view mirror.  Then, while backing, you can align
the center of the inside rear view mirror with the center of the back
seats with the center of the rear window (of the tow vehicle) with the
tape on the trailer.  When everything is aligned, begin backing the
tow vehicle.  So what if you have to stop and get out 2 or 3 times to
check on it.  Keeping the tounge jack handle straigh up helps too.
That is one way to do it.
Another method is to buy 2 round magnets, or square ones, about 1 1/2
to 2 inches in diameter with holes in the center.  These look just
like washers with small holes in them.  Then screw 1/2 inch dowel
sticks in them.  Each stick should be 24 to 30 inches long.  Paint
them bright red or yellow, or one of each color.  Place one magnet on
top of the trailer hitch cup (on the trailer), and the other on the
trailer ball of the tow vehicle.  Begin backing and keeping the sticks
lined up, and with the center of the rear view mirror.  When the two
sticks meet, you should be OK.  A couple of practice sessions at home
will help a lot.
If this is not clear enough, email me, with Trailer alignment as the
topic, so I wont call it spam, and I'll find a web site for you.
Happy and Safe Camping