Hi, Where are the two inch holes? As industrious as mice can be it's very rare that they will chew through that much plywood. The hole could be for something else. Are the holes by the black tank?
Yes, Your Airstream came with insulation in the floor. Think of your trailer as a little home. Do you have insulation in the floor of your house? It's your Airtsream and your choice to put insulation back in.
This is a good thread for blocking mice entrée points.
Huge potential mouse entry on all Airstreams