Large Rodents in 2008 Cougar TT

[126whitetail]

Hello everyone... thanks in advance for reading. Any input is appreciated.

We purchased a new 2008 cougar travel trailer from a dealer about 7 years ago.

We would always remove food from it after every use, however within the first two months we started to get mice in it.
We tried to plug everything we could think of with steel wool, but they kept coming in. To date we have caught easily over 70 mice.

During our most recent trip in Sept. 2014, I noticed a mouse appear on the rear bumper(picture below) then jump up into the TT somehow. Once we got back home we decided to see what we could find, so we removed a wood panel below the stove(see floor plan on reply below), this it what we found... it appears that a large rodent is/was living in there. We are now thinking that the TT is defective in some way? Something seems wrong if a large rodent(much less a mouse) is able to enter?