Well, the internet wasn't much help here. But, if Gulfstream does the same process with your model as ours, you'll find a couple of plastic pipe ends on the 'hookup side' of your unit that are underneath the rig, just a couple ends poking through the belly cover that seem to be doing nothing. The caps are actually the valve ends to drain the plumbing. Open up all the faucets, then open these caps on the ends of these two pipes and that drains them.
Here's a link to some very useful information for us RV types:
http://rvbasics.com/techtips/rv-fres...nterizing.html