Thanks for all the replies. I know everything has a chance of getting mice,these holes are a bit bigger than I expected though. For the shower hole it is only because of the Ptrap, nothing coming out of it.
To access under the shower I have about a 3”x8” panel on the inside. I didn’t find nests in there, but see lots of evidence of mice. The gray water drain goes out the side of the shower on the inside of the trailer and connects to the kitchen drain. So the mice follow the pipe through the shower, under the bathroom sink, and through that wall under the refrigerator, stove, and kitchen sink.
I wonder if we could just use plywood or another material to make a pyramid shape to screw to the subfloor underneath. Then it would be tight and could be removed if needed.