I would use a roller bed down each side of the trailer, but ONLY BECAUSE i HAVE THEM!
Do you have a future project in mind that might need some PVC pipe? As was suggested, find several pieces of pvc pipe, probably in the 4" or larger range and put them on the deck spaced about 2' apart. That would allow the hay to roll, however as the pipe gets debris in front of it, the pipe won't roll worth a darn. The larger the pipe, the easier it will overcome piles of hay in front of it.
You know anyone putting in pipe that might have some drops that they might let go cheap?
Another thought..., and be careful with this one, is to unload them sideways. Now that is going to REQUIRE you to put jackstands of some sort under the pockets of the trailer so that the trailer doesn't capsize or flip. Going off the side reduces the distance you will have to move them, reduces the amount of pipe you have to put under the bales.
Putting a pair of 2x4's under the bales as opposed to another members' post saying use a piece of plywood.
Can you use satelite view of the address to see if there is any trees large enough to use as an anchor?
With limited knowledge of the property before going there, I would probably make it where I was self-sufficient rather than relying on something there that might not work out.
The idea I like best is to mount a winch in the receiver of the truck and use it (along with several chains to extend it's length). You would need to make sure you can pull from the front of the bale so that the bale will come off the bed before running out of chain. Obviously a roller on the back of the bed to change the direction of the pull would be needed. Tractor Supply and Harbor Freight have some nicer pulleys. Don't get the little small one, where the wheel is about 1.5" diameter, because the wheel is cast and will break under tension. (ask me how I know that, and I had 4 spares!! Broke them all before I went back and bought a good one!)
David from jax
Another suggestion, try this at the storage area for the hay bales BEFORE you make the trip to go unload them. A lot easier to "fix" stuff that goes wrong if you happen to have a big tractor sitting there.
David