Are you sure that wasn't a squirrel? Or did you have too many beverages Friday night?
I guess one ( a mouse) could possibly crawl into the rooftop AC housing and find its way into the duct that is formed in the roof foam for the ducted air if an opening was left somewhere. It doesn't take much of a hole.
Climbing the stack is definitely possible. You need to look at the floor of your trailer and identify any openings. This includes vent stacks, water lines, propane and electrical. Seal these holes with wood and caulk with steel wool and construction adhesive. Mice in your roof for a prolonged period could destroy the value of your unit. Once it's all sealed up I would call an exterminator to ensure that there are no mice left.
No overhead trees. And not enough beers to be hearing things.
From inside, would it be possible for mice to access the roof interior by climbing the plumbing stack?
Other than the AC duct and a couple areas where they routered out foam for wiring, the roof on a Roo is solid laminated bead foam with some aluminum supports buried inside also. I doubt a mouse could tunnel through it like a ground mole.
I kept my Roo covered in my backyard over winter. When I took the cover off, I found a lot of mouse droppings on the roof. They had apparently been living between the cover and the roof. There was no evidence that they had been inside the trailer or that they found a way into the roof (and no damage to the cover).
I'm interested to hear what you determine to be the issue.
you are lucky it was mice. had a fifth wheel in storage and when we uncovered it in the spring we found piles of Racoon droppings on the roof next to the A/C. Seems the cover formed a nice tent and the sun beating down on it kept them fairly warm. We could see where they climbed up the inside of the cover. As far as mice I have only seen two trailers in 40 years that mice or ground squirrels have entered the roof. both were through the A/C unit. Plenty of damage to the foam A/C ducts and the light wires.
My brother brought a few mice with him from Colorado in his motor home. They ended up in my pool skimmers quite dead. I have birds that play around in the water on my slide covers and lately squirrels that jump from one tree to my camper to the tree on the other side. The birds pecked holes in my old campers toppers. I'm going to try that mint
smelling stuff on the roof. Birds sound like scratching also.