Use a rope or cable rated higher than the full weight of the trailer. Find a tree and wrap a rope/cable around it. If you have an electric winch, use it, otherwise, use a boat trailer winch and just crank it up. If you have pulleys, use them. If using pulleys, do at least a 3 or 4 loop system. This reduces the load by the number of returns. 1 pulley= full load. 2 pulley = 1/2 load. You must also increase line length to match # of pulleys. You can even use the big, long ground screws used for mobile home tie downs instead of a tree. Just use several and spread the load between them. Have friends with locking blocks so you can switch the angle of the pull if needed. If using a boat winch, make sure it is geared, to make it easier on you.
Using a system similar to this, while rebuilding my house a few years back, and due to some morons who delivered a prefab garage and it literally fell off their trailer, I pulled a 14x28 foot, fully built garage up 60' of steep incline, using steel pipe rollers. I had a 3k# electric winch and used 1/2" cable, attached to two big oak trees, but it worked like a charm.
If you believe you'll be doing this manuever many times, and don't have a tree to attach to, then put in a couple of permanent ground attachment points and buy yourself an inexpensive 1 ton 12v winch. That will pull your trailer up with no problem. You can use the same system to help guide it back down when needed.