Wow, to see this thread resurrected after nearly two years. I now have enough experience to respond to my own question!
We generally continue the open gray valve 'till the night before breaking camp when we have full hook-ups. I close the gray valve the night before after the dishes have been done and all the major water has been used for the night. On the morning of breaking camp my wife and I each take a navy shower and that pretty much fills the gray tank. I then empty the black tank and use the gray to flush the hose. I close the gray valve and use the on-board flush system for several minutes and empty. I repeat a couple of times and then run long enough to ad 3-5 gallons of water in the black tank and disconnect the water and sewer hose. I add a orange pack of tank chemicals to the black tank and off we go. BTW, I have a "special" hose for use with the flush system and a separate white hose for drinking water. Even though the manual says there is no back-flow from the flush system, I have observe water run out when I disconnect the hose.
I always top off our fresh water holding tank so we have plenty of water for last minute pottie use and washing our hands after finishing hooking up to the tow vehicle and for on-the-road emergency pottie stops. It also helps lower the center of gravity which helps stabilize the Airstream (so I'm told) but the trailer tows smoothly even when the tank is empty.
I have also found that the latex gloves break down quickly if stored in the bumper storage but the nylon ones last a lot longer in the storage compartment.