The goal is to get as much water out as possible. There are several places that the water cannot be completely removed, and as long as there is ample room for freeze/thaw expansion, its not a problem.
This is generally how I do my winterization:
The Black and gray tanks must be drained and the drain valves left open. When draining the rest of the system and blowing or pumping water out of the lines, the water will flow into the black and gray tanks and must be allowed to drain out of those tanks.
Here are my key areas that I like to make sure have antifreeze.
City water connection and plumbing.
This line usually has a back flow prevention valve that prevents draining or winterization any other way other than forced air.
On mine the back flow valve is leaking, (probably damaged from not being winterized) I can loosen the outside connection cap, turn my pump on, and water will flow out of the city water connection. This is great, when I winterize and pump antifreeze through my system, I loosen the cap and watch for pink. I know antifreeze is now in this portion of the plumbing.
I drain the water heater (and the rest of the plumbing) , With my pump drawing out of the antifreeze jug, I turn the pump on until I see pink coming out of the water heater drain.
On mine, I now close the bypass valves and run antifreeze to all the working faucets and toilet.
The pump is sucking out of the jug, so it will have antifreeze in it.
A small amount of water will remain in the fresh tank after draining, I dump a little antifreeze in the fresh tank for my piece of mind.
P traps, Sinks, shower, etc
I dump about 1 cup of antifreeze into each sink / drain to ensure there is antifreeze in the p-trap.
Dump a cup of antifreeze into the toilet bowl.
BLACK and GRAY tank
Leave the dump valves open, anything water that is inside will drain out.