You will NEVER get all of the water out!
Drain, shop vac, then reverse shop vac and blow RV anti-freeze into the lines. If you haven't done this in the past, you could just be getting very lucky.
RV or pool anti-freeze ONLY!!!! If you use automotive anti-freeze you will be soaking in poison when you fill it next spring.( I know this seems obvious, but I've seen it done)