A hole big enough for them to squeeze thru is how they get in, but I've parked my trailer about 3 feet from my wood shed for the past 10 + years and have never had a single rodent in the trailer...
Here is a pic of how close it is.. The white lattice thing behind the trailer is a 16'x4' wood shed.
I put the dcon trays in the wood shed and they are empty every couple of weeks...
I find some carcasses in the wood pile from time to time, but with as much as they eat, they must die in other places too?
My Dad parks his 5th wheel inside his huge shop and has gotten rodents in the thing to the point he had to replace all the carpet in it... He sets traps when they seem to be present and gets bodies all the time.. He sets the dcon trays around his shop too, and they empty as well...
This is a $40K RV and my little TT cost $14K brand new and sits outside and has never had one in it.. :)
Anyway, good luck with it!
Mitch.