Our dog got a puncture wound and a 2 in. long slit in his skin from something in the back yard. This is a fenced in back yard. We've checked the fence for protruding nails, the swing set for protruding screws and all that's left is a trailer with some brush on it. Possibly he went under the trailer and snagged himself on the metal.
Could you empty the trailer and then turn the trailer on its side, so you could see if there are sharp edges or protruding screws? Then you could fix the sharp places. Or you could just turn the sharp side toward the fence. Or you could put the brush from the trailer under it as a barrier. I hope your dog recovers quickly.
I'm sorry about your poor baby. That must have really hurt.
You also can get cheap chicken wire fencing & just fence it in.