You're on the right track. They can be kept out by diligent hole-filling. Search out all wiring and piping entries underneath. Spray foam them if you can. Side wall holes shouldn't be a problem unless there is a way for them to climb to the hole. A power cord is obviously a way for them to climb.
Don't forget that steel wool rusts. So if it is exposed to the weather, you'll have a problem. I think there is stainless wool?
I got some scraps of galvanized tin (you could use aluminum too) and cut inserts and covers for things like the power cord hatch so I could use the cord but not have any opening for mice.