They are I-beams that a generator was mounted to. They look like they should come out easy enough I'll play around with it tomorrow in the daylight.
The I-beams are bolted to a steel plate.
The plate is attached to the trailer using u-bolts through the trailer attachment points, 4 on each side.
The u-bolts are covered with a channel to allow a fork lift to lift out the entire assembly with the generator on it.
On top of the channel is an aluminum cover which is also bolted to the steel plate. The aluminum cover has to be removed in order to get to the u-bolts. When I went to remove the nuts to the cover, the bolts, the heads of which are on the bottom of the steel plate and inaccessible, turn with the wrench. The bolts holding the i-beams to the plate will present the same issue.
I am going to try a nut cracker to break off the nuts. I would grind them off with a cutoff tool but the clearance is too tight. A Dremel tool will fit however.
The entire assembly with the beams and the steel plate is sized to slide out the back with the tailgate off.
The two beams and the steel plate weigh several hundred pounds.