I realize this thread is primarily about rodent issues in front of the firewall, but there's a potential for interior intrusion as well, as one of the recent trailer threads confirmed.
On that trailer thread, it appeared as if some folks had never heard of copper wool. We use it in the south to prevent rodent penetration of slab-on-grade homes. The two most common applications are brick facade weep hole stuffing (some improperly-constructed weep holes are wide enough to allow mice in, but the air has to keep flowing through those holes, so the wool blocks the mice while still allowing ventilation) and packing of the wall penetration where the A/C lines extend to the exterior components.
This past weekend, I took inventory of our Interstate rear of the engine compartment. My husband had foam-sealed all the penetrations for the gray water system when we replaced that plumbing last year
, and I saw no evidence of chewing, so I left that work alone. The only other area of concern was at the black tank line, where a large gap exists (why Airstream cut it that size is a mystery; the shower drain opening is more befitting its pipe). The pic collage shows the wool stuffing process. This penetration is also where my husband runs any lines that have to go under the chassis, so it's best not to seal it with foam. If he needs to run another line in the future, he can take a needle-nose pliers and tug the copper back out.
I don't know if this idea would have application in the other areas of the Interstate where rodent problems have been reported. Remember, copper is superbly conductive, and it might cause an electrical short if used in the wrong way.