I would use a winch. Actually I have done this to pull my old pop-up up into a garage. The garage was off an alley so I could get it close with a vehicle but then turned it 90 degrees by hand and connected to a boat winch. The incline was too great for two people to even budge it up the grade. Used 12v power from the truck and up it went. You may need a stronger winch and stronger power source but very doable. I had bolted the winch to the concrete in the rear of the garage. If your trailer will remain on an incline I would also create some type of fixed hold down so that it does not move.
winch was something like this:
Amazon