We've had a fair amount of success since a family of feral cats has taken up residence on the MH chassis for years. We have found a few mouse carcasses, but no evidence of infiltration into the MH. Peppermint oil is a reasonable deterrent. You can also use spearmint oil, and/or eucalyptus. The camphor in the moth balls is also unpleasant to most rodents. I would heed the advice of other posters, but would use the oils in conjunction with the sonics, runway sticky traps, or all three.
Here, in NY, we have had a number of cases of hantavirus. The pulmonary strain of the virus, usually transmitted by contact with rodent feces can be fatal. Better to take the extra precautions, and err in the favor of safety. Just my opinion.