I agree it is really nasty to reach into the recess and clean out a nest and all the droppings. I was also worried about mites so I wore gloves for that chore. Starlings loved that area and they are very messy birds.
Then I used hardware cloth, 1/4" grid, to block off the pin box. This is stiff enough that I could bend it to fit, and stay in, the area between the front of the hitch and the bottom of the trailer. I also cut a piece to fit between the top back of the hitch and the trailer. These pieces stay in place all the time. I store the coiled umbilical cord in the hitch recess area when not in use. So I cut a third piece of the hardware cloth slightly larger than the opening and put it in the back opening of the hitch after I place the cord in there. When we are on the road, I put this piece of hardware cloth in the storage area and put it back in after we get home. This has kept the hitch area free of birds now for 5 years. The trick is to cover all possible access points.
What really works is the plastic life size owls. You fill the bottom with sand and can be found at Ace Hardware. No bird or rodent even thinks of going near them, darn things really work! We placed it in the middle of a yard to keep a hawk from getting my friends baby chicks and he'd fly in see the owl and take off!
Not my experience.
"We must be willing to get rid of the life we've planned,
so as to have the life that is waiting for us".
we had that issue with our 25.5K Terry, unfortunately our nesting starlings got out in Oklahoma City, they were tired of traveling, when they got out we had another issue, bird mites came into the bedroom area and got all over us... we didn't sleep well but three cans of fogger took care of them the next morning!
I used that canned foam and filled the void! Worked great and if you need to get into the area you can remove it with a knife.