MOTH BALLS! I hear people telling me it doesn't work, but in 47 years of motor homing in 7 different coaches and staying in hundreds of varying locations including storing my coach in a horse barn with horses being fed daily while the coach is sitting idle, I have only had perhaps 4 mice enter and found 4 locations that I missed. I agree that the service bay is the most common entry location for them to enter given the demons just walk up the cable, water hose or drain hose so I make the holes as small as possible as shown by JDCrow then use the convenient bag of moth balls that come in the box and cover any holes remaining so that they are repulsed before they try to enter. They follow their nose looking for a place to eat or nest. I usually put a couple of bags in the engine compartment, the generator compartment, on top of the fuel tank and alongside of the fresh, black and grey water tanks, and alongside the battery bank. I find the smell doesn't get into the living area of todays coaches the way it did in my 1970 Banner or 1976 Kings Highway or 1985 Travel Craft so I don't need to remove all the moth balls when I hit the road. But even in those old motor homes, I seldom had issue with rodents except when I found a location that I missed.
Whatever you do, DO NOT USE POISEN! If they do get in and eat the poison, they will likely die inside the coach and the smell may never disappear even if you find all the dead bodies.
I have always had antique or classic cars and use the same method when storing them. It works for them although the camphor smell takes some time to dissipate in the car and sometimes I am putting the car back into storage without completely clearing the smell. I find it not bad and definitely better than rodent smell. I have never heard of any bad health effects from breathing the smell. My relatives used to have winter and summer rugs for their houses and stored the rugs with moth balls every season and I grew up smelling the camphor with no ill effects. I am 78 years old and all of my family, parents, grandparents, siblings and aunts and uncles lived to ripe old ages with no ill effects from smelling camphor.
I hope this helps
Happy Motoring,
Art Perrone