I agree that sealing all the entry points is the solution. Smelly things and other tricks might help, but keeping them out is the real key. That said, it is not easy to find all the entry points. It usually takes spending some time under and around the trailer with a very detailed inspection.
I've learned that sealant methods are critical to keep out those who already have established a happy home. Like many humans, rodents are curious, but basically lazy. They will chew to get back in, but not so much if they are just poking around looking for a new home. Any good sealant keeps them out in that situation.
Breaking into areas for access was mentioned. A possibility is to use a marine deck plate to get access where needed and maintain a finished look while providing future access. They come in various sizes and colors.
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