I have that pre plumbed washer line for future resale. Never used it. For 2 winters in N Georgia mountains I do same as stated.....the faucets, hot/cold are brass. No water has ever flowed up to either. With all low points drained; air blown and about 2 gals of RV anti freeze, never any concern.
Since I have the main filter system intact; and; I do use the RV in the winter, re-winterize, I remove the filter element and take it home and stow it in a refer in a sealed zip lock bag. The system fills the clear " plastic canister" with anti freeze and the seals all get lubricated. I then pour the canister filled anti freeze into the shower drain and bathroom sink.
2 years and nary any issue....
In 5 years of winters with fifth wheels,
Only thing EVER, was a cracked ( and proudly spewing) PLASTIC water intake to the toilet. That was because I blew with air and did not use anti freeze. These new units makes the task more simple. TAKE YOUR TIME!!
I am a firm believer in ANTI FREEZE.....