Under your mobile home
The “belly” is what keeps your water, sewer, and electrical lines away from the weather, bugs and vermin. It also provides a way to keep the heat/cold from your furnace and A/C ducts contained within the house.
It is frequently damaged when water leaks soak the insulation and tear it loose. In fact, one of the key tests before buying a used mobile home is to look under it to see if there are pieces of insulation/belly hanging down. If there are you can be certain there have been plumbing problems at those locations.
Mice can get through VERY small holes. Being intelligent creatures they like finding a living space that is dry & warm. I guess they don’t itch as much from the fiberglass as I do. Unfortunately, they also die there. That’s one of the times people write and ask what to do about the terrible smell in their house.