Is the bottom just sagging but otherwise in reasonable looking condition? If so you may not have to completely replace the belly pan. It is typically held in place with aluminum pop-rivits of a type that you get at Home Depot, Lowes or most Hardware stores. Sometimes what happens is that the old rivits may have pulled through the aluminum of the belly pan. If that has happened you could drill out the old rivit and either put it a larger one or even add an aluminum washer big enough to cover the hole and use the original size rivit. You can usually buy washers in the same place that you can buy the rivits.
Some folks in these forums have also had good results using stainless steel sheet metal screws to hold their belly pan in place.
As to the issue of getting rid of rodent activity - that may be a bit more work. It is not uncommon for units that have been sitting out for a while to collect rodents in the below floor area where they evidently like to burro into the fiberglass insulation. If you are pretty sure that has happened you may have to take the belly pan off and remove/replace the fiberglass insluation. While this is not terribly dificult in principle it can be a dirty job. Even without rodent infestation the underfloor area can accumulate a lot of dirt. It is a good idea to at least wear a dust mask when you are taking off the belly pan. I understand that if there was rodent activity it is a good idea to spray all the exposed areas with a mixture of bleach in water before you put things back together.
The rodent issue can be really bad news since there can be an issue with the hanta virus which is sometimes carried by mice. There are also a lot of thread in these forums about mice. One to check is titled: Mice inside trailer. You can find this one by clicking on the following:
Here is another thread:
A thread about electri mice repellent:
If you click on the "Searc" button at the top of this page you can type in the key word "mice" to find other postings on the topic.
I hope this gets you started about thinking about what to do. If you have other questions feel free to post them and maybe someone here can answer them.