I agree with all advice above and will add some more.
My first winter trip I drained the outside shower, but not the inside shower. In my 2900D the shower plumbing is in the outside wall with the city water connection. It got down to 17F with a north wind hitting that wall over night and the shower plumbing was frozen in the morning, so I would/should assume the city feed plumbing was frozen too. After the sun rose and temps got above 25 to 28F it thawed and no leaks.
The next night I slept with the bathroom door open, AND the low dropped to 12F outside. The shower never froze. - - albeit the shower water was COLD in the morning and it took a while to warm up!!! Brrrrrr the next couple nights we're 10F and nothing froze. Lesson learned.
With the door closed I would say the bathroom was about 50F, and with it open it was 70 to 72F.... That was enough to keep the plumbing in the outside wall warm.
I've thought about how to force heat into that wall. It's open to the space under the fridge (next to the shower) and I could put a 12V computer fan and force air flow... But I doubt if I need to prepare for winters below 10F so I never added the fan. I also improved the furnace duct to the bathroom that is routed through the floor aluminum framing. It was leaking air to the outside and wasting (a lot of) heat from my furnace and causing it to run more frequently/longer. Btw, that was after the 10F nights

. So I'm probably in better shape now.
Larry