Like Don said, you typically don't "winterize" your waste system. If not in use, you want to winterize your fresh water system by getting all of the water out and drawing some antifreeze into the lines. A small exception is you want to dump some antifreeze into your traps (sinks) so they don't freeze.
When water freezes, it expands. If its in a confined space like a pipe, that pipe will burst. But a little bit in a holding tank (black or gray) shouldn't hurt anything. I always dump a little antifreeze in the bowl, black tank, sinks and shower just for fun.
BUT, if you're living in it then you're presumably going to be using the water system. That introduces some complexities. You're going to want to keep the rig heated - at least at or a bit above freezing. If you have an external hose (city) water connection, you're going to have to keep that from freezing somehow. My RV is designed in such a way that all of the water pipes run through areas that get radiant heat from the furnaces so they don't freeze. Waste tanks are underneath and waste can freeze and prevent you from draining the tanks if you don't add some antifreeze. Again, this assumes you're using them.
As far as location goes, just follow the plumbing off of the dump pipe. It's usually connected to the black tank and a smaller pipe will feed into it from the gray tank.
