only so many ways this can happen.
The fill valve is faulty and the water pump filled the bowl. You said the fresh tank is empty so this probably isn't the issue.
The fill valve is faulty and shore hose had extremely high pressure and filled the bowl. This is a possibility and you should be able to reproduce it fairly quickly.
There is a crack in the skylight and the bowl filled with rain water. I find this unlikely as I don't think Paso Robles gets enough rain to fill the bowl through a cracked skylight.
Which leaves the last possibility. You said you like to clean your tanks really well. My guess is that you lost track of time while filling the tank for a flush and the water pushed up through the fill valve and filled the bowl. Then you pulled the drain valve handle and drained the tank but the toilet stayed full of water. When you next moved the trailer the water in the full toilet sloshed over the rim and landed on the floor.