I back a 38 foot class A in solo from time to time, Basically I have been using reverse both with single units and trailers since long before I ever drove a car (Farm boy) so how I do it may not be the best of ideas.. Though I will tell you of one campsite (later).
What I would suggest is you get some collaspable "Dunce Caps" (Traffic cones)
Put one on top of the sewer inlet and put a few in other places with wish to note.. Keep them in your mirror,, Put one somewhere you can use as a marker spot as well,
Now that one.. There was a paved wash directly across from it so I pulled forward onto the wash, Popped it in Reverse and straight back onto the site.. Impressed the first time trailer puller in the next site it did, but as far as backing in.. Well it don't get easier than that :)
