What JimK said.. I've parked my TT's in the same spot for 20 years now. I have a firewood shed right behind it on the side of my house. I put rat bait in the firewood shed all the time, and find the remnants of them all the time. In addition to all the turds and chewed bits of firewood so I know the area has rats/mice present.
To date, I have never had one in a TT. I have never placed any kind of deterrent in the TT either.
Only thing I have done is make sure there are no points of entry from the outside of the trailer. Especially the under side.
Even the smallest of cracks or gap can let the little varmints in. On the first trailer, I did seal up some very small gaps with silicon sealer. On the current trailer, I've not had to do anything to it. I did inspect it when I first got it of course. That meant crawling on my back under the whole underbelly. This one is a 'sealed' underbelly, but it looked like the manufacture took care of any cracks/gaps before I got it. :)
Anyway, I'd be doing a close check on the underbelly and seal up any cracks/gaps before using any of the so called "repellants".
On a side note, I have heard a lot of folks say all they do with the dryer sheets is make a bed with them.. :)
Wood shed is in background.
Good luck! Mitch