Hi Neighbor:
I don't think you could find anybody with winter conditions like Maine, unless you looked north (where I am)
You already know about the water system. Remember to get all the canned goods, shampoo, tooth-paste etc. out as well, as you know any liquid or paste type item will freeze and burst leaving a mess.
I have always stored outside and covered but this year, for the first time I will be inside an unheated building.
Change your engine oil and filter so the engine is stored with fresh oil. Grease all zerks you can find. Check your engine anti-freeze and make sure it is good to -40. (Make sure your windshield washer fluid is winter grade as well) Inflate your tires to your normal travel inflation. Fill your fuel tank to prevent condensation.
I have a gas engine so I use a storage additive. I think there is a cold weather additive for diesel that prevents it from gelling during extreme cold. Check with the engine manufacturer for recommendations. If you do use an additive run your engine and genny to get the additive through the entire fuel system.
Top up all fluid levels like power steering, transmission, brake etc. Charge the batteries fully.
Wash the rig as dead bugs left to sit on the paint or clear-coat over the winter can leave a stain.
As for the batteries, I used to remove them and store them in my garage. The last two years I left them in the rig with both terminals disconnected. My experience is that when I hook them up again in late April they are still charged and the engine will start. I would however prepare for the worst and take booster cables with me when I retrieve the rig in the spring!
Many people recommend starting once per month or so during winter but if you are in an enclosed area that may not be possible. There is a school of thought that thinks that sub-zero starts with the engine oil as thick as molasses are harder on the engine than letting it sit. The engine will not really get up to operating temperature unless you drive it. If you drive it then the salt on the highways here in the winter will do damage.
Put something under your tires to keep them off the ground or concrete. I will be on concrete this year and plan to use plastic cutting boards from the dollar store under my tires.
I will cover the rig, even inside. It keeps it from getting covered by bird droppings. If you are inside a cover will not blow around so no worries about scuffing the paint. I plan to leave the roof vents cracked open a little to keep the air inside the rig fresh.
In spring you are ready to go and start camping. Your tires may need some air.
I hate storing for 6 months but that is reality here. Remember that during that cold February weather, summer is just three days away, straight down I95!
Safe Travels