Mice are a tough nut to crack. You need to find the entry point(s). Gaps around pipes can be filled with stainless steel scrub pads. I use Aluminun screen across fridge
vent openings. etc. You get the idea.
Just about every car made has an open door to mice under the cowl where the fresh air vent is. Once they're in your car the only way I have found to get rid of them is snap traps baited with peanut butter. If there is any chance they are still in your trailer, that's what I recommend. Car dealerships love mice. They make thousands repairing the damage, in particular car heating systems. Subarus and Hondas are particularly vulnerable. Do a search. You will be amazed at how big the problem is.
This is from a previous thread I started on the subject
Two sites I found helpful. The first is the cliff notes version. The second, the "Moby Dick" version.
Control of Mice | University of Kentucky Entomology
Prevention and control of house mice, Mus musculus
The thread is here Is Mickey living with?
Good luck, Raz