Blow out the black tank flush as well and the outside shower.
After i use gravity to drain the freshwater holding tank and use the low-point drain to partially clear the water lines,I blow some air thru my system to get rid of any more hidden water which collects at the elbows.
I then pump antifreeze thru the system so it does not mix and dilute with residual water in the system, i feel this gives the full benefit of the antifreeze and it runs thru the water pump as well.
I then blow out the antifreeze because the system is now protected fully.
There is NO need to leave the antifreeze in the system, i flush it so it doesn't leave antifreeze residue in the lines.
When springtime comes around again, i just have to fill it with water and im on the road again.