[JRTJH]

We know very little about where this Cougar has been stored during the past 8 years, nor do we know what the OP has done (other than use steel wool to plug some holes). Knowing much more about the place it's stored, how long it's been left uninhabited, what has been done to seal the access holes, and if any cleanup/actions to stop the infestation have been taken.

Right now, all we know is that a 2008 Cougar has a rodent infestation and more than 70 have been caught in the past 8 years. We don't even know if the owner has made a thorough inspection of the underside, to include a careful search for any areas where mice could have gnawed their own access through the coroplast and into the trailer.

We just don't know if this RV was parked on the edge of a cornfield and left for 3 or 4 years and "now has issues" or if it's been in a busy storage lot, used every weekend and diligently worked on to try to resolve this problem and nothing seems to be working...

Granted, it's a mess, and an unsavory, nasty one at that, but much more information is needed to make any judgment about what may be the source of entry and what might be possible solutions. Anything at this point would just be a guess as to why it happened over the past 8 years, or how to go about fixing it.

ADDED: The OP stated: "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" but he doesn't say if he found any area where this mouse gained access to the trailer or if he "just saw it on the bumper and then go into the trailer"..... Was the entry hole found? Was it sealed? Are there other entry points nearby?