Open door.
Insert this.

Profit.

I read an article a few yrs back saying none of the  'ultrasonic' pest stuff works..

iMHO - the best bet is to find out where they are getting in and then fill the cracks with stainless steelwool..

Brian

At least there isn't a moose in it!

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Mothballs seem to repel them but that's supposedly an incorrect use of them

.

I just went through a similar problem but mine was in a popup camper that was sitting just off my driveway.  We were too busy to camp this year and I opened it up and saw 2 little mice just staring at me.  Luckily, they only got into the paper towels and toilet paper.  I killed 8 mice in 2 days with traps and then went to work on preventing them on getting back in.  

I jacked up the camper and then caulked or foam sealed every crack and crevice that I could see.  Pay attention to where water, propane, and electirc lines enter the camper.  I used copper mesh to plug any big gaps and then caulked over it.  I also put a few of the little green bait things near suspected entrances.  Before doing this make sure all entrances are sealed as you dont want dead mice hidden in the camper.  Of course, keep the ground clear around your camper.

Do you keep food in the camper?  If so, reconsider doing so and only bring what you need for the trip.

ETA: I saw it has slideouts.  Can you just keep the slideouts open and then seal around them?

One more tip that some campers swear by is using dryer sheets to repel mice.  

I don't think they work and that the people that use them keep very neat campers to begin with and just keep it so clean and fresh smelling that it doesnt appeal to mice in the first place.  It can't hurt to try it and your camper will smell better.

This.

Same thing with Trailer homes( older than 4 years) The manufacturer does not seal the pipes where they enter the bottom of the home. Mice get through there all the time.

Steel wool works wonders, They hate that stuff.

I have mice in my fucking house this winter and I am going fucking nuts trying to evict their asses. I am up to 11 fucking dead mice and I keep killing the bastards. I would use poison but my stupid dogs would eat it or at least chew on the dead mice when they found them, I would get a cat but my dogs would chew on that as well. Fuck it I just want to burn this fucker down I can't stand fucking mice.