Mice in a van are terrible, and I've been concerned for a long time about possible ingress points, especially above the fresh water tank under the galley. I finally got around to doing something about it. I cut a piece of hardware cloth about 6"x10" and covered the opening securely. I had to loosen all 4 screws on the bracket that holds the junction box, but otherwise it was a 20-minute job. To access the space, I took off the vent cover near the coach power switch, and I took out the lowest drawer.
(Note: I used hardware cloth because I thought it would possibly be useful to have warm air from the cabin circulating around these non-insulated pipes in a cold climate. Update 2021: I decided to go ahead and seal this area off due to excessive road dust coming through here. See this post.)
26-August-2020