Unlevel surface is ok as long as truck and trailer are on the same slope. If that's the case, just measure the trailer frame front and back and get it the same distance off the ground. Then back the truck right next to the coupler and set up (about 1/8" for ever 100 lbs TW higher than the coupler seems to work well).
Now, if I'm picturing your situation correctly, you can't do this and will be pointing nose down on the trailer and even or nose up on the truck. That being the case, set the trailer up as I said before measuring from the frame, then measure your coupler height and set the hitch accordingly. Unfortunately, you can set the height like that, but you really need the entire rig level to properly adjust it.
2012 Chevy Tahoe LTZ - HD tow package
2017 Rockwood 2703WS Emerald Edition