There is only one way to know what to do. Get under the trailer and look at what is open.
In the exterior of the belly pan metal:
Caulk small places that are large enough for a mouse (any water that gets inside still needs to weep out so not water tight)
Re-rivet loose metal
Stuff large openings with brass wire wool, putty tape, or similar material.
Install coarse metal screen wire over vents that are required to stay open (like a fridge vent thru the floor)
Do not seal air tight around openings in the interior wood floor. They are needed for return air from those ducts that go into the belly pan.
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FOR YOR SAFETY wear safety gear when you get into cleaning up, read here:
https://www.cdc.gov/hantavirus/