Rodents (mice) keep infesting trailer and pickup truck ???

Mice want a few things. A dry area and close access to a food source and a tiny bit of water.

Old fashioned moth balls will work, but you won't like the smell.

I put dcon under the hood of Subaru it seemed to work haven't found anymore nest in the heater vents.

You can try spraying a product called Mouse Oust (available online) on the ground outside of the trailer/truck. We have had limited success with it but my MIL swears by it. It has a fairly pleasant mint smell. Other than seal EVERY and I mean EVERY hole, no matter how small. We just had to scrap our house fridge because one crawled up inside the back where we couldn't reach and died.

I'd try to make sure to fix any places they have been getting into to trailer. Then placing the large plastic outdoor rodent bait boxes under the truck and trailer. If they can feed outside before trying to get in the trailer they should die outside and not inside stinking up the trailer. Make sure not to store any food inside the trailer and wipe down and clean the microwave, stove, stove hood (including the filter if it has one), counter tops, sinks and cabinets where food has left any residue between uses.

check with local feral cat rescue/control

some groups catch them spayd them then turn them loose

a couple of spayd feral cats will get the rodents

with out breeding a population that will get out of control

You need to find and seal every and I mean EVERY hole. SS screen followed by spray foam. Great Stuff has one that is listed as rhodent proof. Does a great job of stopping them.

If you can't be there to manage a bunch of peanut butter baited traps then give them Dcon while you are gone.

Get a couple of cats.

In the area of Chicago there's a big problem with rodent control. Daughter, son in law, and grand sons live in Chicago and had all the wiring on their Toyota replaced twice due to rodent damage. Dealer said Toyota uses soybean spraying on the wiring for some purpose.. A bounty or some birt control is necessary to thin the heard.