The water lines need to be free of water.  They do not need to be full of antifreeze - antifreeze just ensures you have displaced all of the water, which has happened once you see pink stuff come out of every faucet.  Then you can drain the antifreeze, but most people just leave it in there all winter.  No worries about opening up your low-point drains (which have nothing to do with the sink and shower drain).

When you pour antifreeze into the drains you are displacing the water there, too.  You could go through the trouble of removing the traps to pour out the water, but since you need to maintain the "airlock" created by the liquid in the trap, you need something in there to keep the gasses out of the pod, so it is antifreeze.  

Idaklee - don't worry too much.  Yes, blowing out with air first is optional, but you end up using more antifreeze if you do.  As mentioned above, open up your low point drains and you'll be ok there.  The dealer only using 1/2 gallon, especially on a slide-out model which has more pipe, is something to worry about BUT some of that is probably because he forgot to put some down the shower and sink drain traps.  You can fix that by buying a gallon of it and pouring a cup down each drain.  It doesn't account for all of the missing antifreeze, though.

On my 171 I used a little under a gallon each time.  I used maybe a gallon and a half on my 30-foot Salem. 

Hi Marcia,

To answer your question, drain both the low water drain lines.  An empty line can't burst.

Lots of people here are more qualified than I am, and I also own a 2013 177. 

It takes me a gallon and a half to run all the lines properly, could probably do it with a gallon if I was really professional, and it only takes me 10 minutes. 

You can crawl under the driver side wheel, in back of it, and drain both those low water drain lines.  I'd sure do that if I were you.  It's easy.  

 I question the quality of this dealer. 

This web site should start a "Name that dealer" link for experiences just like this. 

Maybe the dealer blew it all out with air, added the pink stuff for the pump and drains.  But the low water drains don't blow out unless they are open, and still might have pure water and freeze and burst. 

Good luck!