This is total fake news. You obviously don't have any real-life experience with mouse-proofing. Ultrasonic devices are proven to NOT work. As are, dryer sheets, soap, hot peppers, moth balls, fresh cab, etc. It's pure bunk.
Consult a channel like Shawn Woods on YouTube to see what works. Lots of other resources too.
The ONLY thing that will keep mice out is to plug the holes with steel, brass, or copper mesh, and foam the holes closed. It is possible, I have done it myself several times, and just did it with my new camper. You just have to be diligent with searching out all of the entrances to the trailer.
I have spent a lifetime fighting mice entering my cabin and my campers. I know what works and what is waste of time. There is no magic solution. Only elbow grease.