Whoever got into mine got in through a 14-inch roof vent. Only one person needs to be small enough, it is a bit more than 19 inches on the diagonal, a few decades ago many interior doorways were just slightly wider. Once someone gets inside, then the door gets opened.
Hooked up RV in a rural area (or an empty summer cottage) is a big temptation for homeless looking for a place to squat in the winter. A lot easier to be unnoticed in a rural location than an empty house in town, and they move into those as well. For several years I kept unknown homeless persons out of a vacant house by putting a homeless person known to me into it.
Waste tanks and plumbing generally don't need to be winterized, beyond emptying and putting in a little antifreeze to keep valve seals from drying out, and to keep traps from going dry. It is the closed water supply plumbing that is most vulnerable to freeze damage.
I responded to the squatter breaking into my motorhome by moving it to a more secure location where the storage property owner actually watched what goes on in her facility.