Same Thing on 2012 35QB4!
I found the same issue on our 2012 35QB4. We had never had a rodent issue at our storage location but as soon as we bought our new fiver, we had a mouse get into our front garage area, and it accessed it through that very location.
For $200, we had our local dealer install a thin steel plate, which they riveted in place, and problem solved. Thankfully, it stopped the one mouse in the garage area from setting up shop and bringing in all of his furry friends! We have not had any further rodent issue as a result.
If you are handy with the tools, it would be pretty easy to do this yourself, and it would just cost you the price of some thin steel (or tin) and some rivets.
If you are interested, I'll find the pictures of the final result - the dealer did a great job, and it looks like it came from the factory with the plate installed.
Neither I or the dealer found any reason for the hole, structural or otherwise, and in fact, the dealer said that it was just a design flaw. The battery box has a separate vent hole, and the battery box is sealed so "rodentia" cannot get any further than the battery.
I felt much better once the "fix" was put into place, and like I said, no more rodent issues! We had stored two separate travel trailers at the same location for four years and never had an issue, until we parked our new fiver, with its landing gear "roadway" directly through the "ingress hatch" into the front garage!
I recommend that you make the fix before you have an issue...
If you want a picture of the fix, PM me and I'll find it and send it off to you.
2012 Cedar Creek Silverback 35QB4 "Silver Edition"
2011 Chev Silverado 3500HD Dually Duramax Turbo Diesel - Allison Trans / PullRite 20.5K Super 5th Wheel Hitch & SuperRails / Trailair Tri-Glide Air Ride Pin Box #21,000 /
Honda 3000i Handi