If you don't have two strong helpers, you could hitch up the trailer but keep the landing gear down. Unbolt the king pin while still attached to the hitch, then raise the trailer using the landing gear. Once at the correct height, reinstall bolts and torque to specs. I've never done this myself but I have heard and read about this method many times before.
2016 North Point 377 RLBH, with a few mods, disc brakes, shocks, Sailun g rated tires, wet bolts
2014 Ford F350 Platinum sold
2017 Ford F350 Lariat, CC, 6.7PSD, DRW, Trailer Saver BD3 hitch