Mice/rat repellent

Its getting to be that time of the year when field mice etc. end up looking for a place to spend the winter. I live out in the country and last year, must have killed (spring traps) 20-30 mice by the end of winter. I have seen one or two rats in my garrage and want to keep them out. I've plugged as many 'holes' or access pioints as possible. I live in a trailer and it is hard to 'seal up underneigh the trailer. Any thoughts, home remedies etc to repell or kill rats and mice. I've heard pros and cons about whether moth balls work.. Any thoughts? Tweet