Why not just drain the entire water system? That's what we used to do. RV antifreeze in the traps and toilet bowl after it was bailed out.
If you locate your drain properly at a low point, the water will siphon out of all the pipes when you open up all the faucets. We had a crawl space under the camp though, so it was easy to set it up so the entire system would siphon dry. If you can't properly drain it this way use compressed air and blow the lines out.