While I don't have a San-t-flush, I'd have to say, yes, you need to at the very least blow it out with air. You could always take a piece of hose and a small tipped funnel and pour antifreeze into it.
Since it has a back flow valve that may require a small amount of pressure to open you could adapt a plastic bottle to the hose instead of the funnel and use it to force the antifreeze into the san-t-flush tubing.
Salem 29RKSS Pushing a 2500 Mega-Cab CTD!
Gotta go campin!