blow out the lines then flip it on its roof so the traps drain out then put some pink stuff in the traps and toilet (don't use it after that)
I leave my dump valves open, after having done a very thorough flushing on the tanks, emptying your tanks will not cause odors to get into the trailer, emptying your P traps will, that's why you put anti-freeze in them.
I had a friend left his closed and some water was still in the grey tank(probably remnants and what came out of the traps) and not enough antifreeze, and he dewinterized in spring to a split pipe at the dump valve.