If you have an air compressor, you can simply blow the water out of the water lines and forget about adding antifreeze to the water lines. Many of us do it that way.
All you need is the two devices shown in the attached picture. The bottom device screws into your water fill connection and allows the top device to attach to it (so no one has to hold it).
Set the air pressure to a constant 40 PSI and attach the top device to the air compressor hose and to the bottom device and open the faucets one at a time to drain the water out. If you've already dumped the black and gray tanks, put a bowel under the faucet to collect the water and dump elsewhere. Don't forget the toilet and the outside shower. You can also open the fresh water tank drain to blow a little more water out of that. When complete, add some RV antifreeze to each drain and you are done.