From what you describe I wonder if they are in (whether inside your living space or simply inside the walls) and now they can't get out. Why do you say they are inside the walls? Inside is inside. If they are inside the outer sheet metal of the Mercedes Benz envelope they are in. Once they are in, they will go wherever they want. Why do you describe the scenario as inside the walls and then go on to say inside the skin? Maybe I don't understand what you've described.
There are plenty of places mice can get into any vehicle. Cabin air filter duct. Wiring ducts and plenums from the engine compartment. In a vehicle such an Interstate there are any number of holes that Airstream cuts through the floor, walls and roof that could be entry points. It sounds like you've looked these over and "plugged" them. Plugged them with what? Not to be argumentative but you have to look, look a second time and then look again for any and every hole or crack that may be allowing mice to enter the vehicle. Every wire, every hose, every piece of plumbing that pierces through the Mercedes Benz envelope is a potential point of entry.
We've only had a mouse once in our trailer and once in our van. We were able to catch them with the peanut butter mouse trap arrangement. Good luck.
Lucius and Danielle
1992 29' Excella Classic / 2010 Interstate
2005 Chevrolet Suburban K2500 8.1L
2018 GMC Sierra K1500 SLT, 6.2L, Max Trailering
Got a cooped-up feeling, gotta get out of town, got those Airstream campin' blues...