Scott, if all you have is a city water line to the sink, and a single faucet, I believe all you would need to do to drain that line would be to remove the hose, open the faucet and leave it open. Go outside to the port where your hose screws on and examine it. Remove the gasket and inlet screen, and you'll see a pin in the center. Move aside and push the pin in. A small amount of water may shoot out under pressure. Hold it open until fully drained.
If you look in your kitchen cabinet below your sink, you'll see a clear vinyl tube connecting your sink drain to your gray water tank. It will have water sitting in the lowest part of the line. If you can see the entire line; just push up the sag in the line, and let it drain into the gray water tank. If I was you, I would buy a gallon of RV antifreeze and pour some (a quart?) down the sink drain just to be sure. It is cheap insurance and won't hurt a thing. Finally drain the gray water tank.
I would make absolutely sure the fresh water tank has no water in it or the feed line to the sink. You said "new", Scamp
, so I'm assuming the tank was never filled? Scamp uses air pressure to test the plumbing system, so you would know if the tank was ever filled.
Hopefully if someone thinks of anything I missed, they'll weigh in.