A few years ago we had the pleasure of having the seal on the black tank roll over and tear while we were out dry camping for a week. As a result, the knife valve not only didn't seal but it walked open and jammed. The result was NOT pretty with a week's worth of stuff. However I learned a long time ago that there are 2 kinds of people: The quick and the wet, so very little got on me.
Do I understand correctly, GraciesMom left the black tank valve open and relied on the cap on the end of her sewer hose?
The cap on the sewer drain pipe--or hose if you leave the hose connected to the drain pipe--is not the primary device that keeps the stuff in the tank until your completely ready to dump. After our fiasco, my technique is to loosen the drain cap but NOT remove it until after I've felt for the pressure of stuff on the inside wanting out. If there is no feeling of pressure, then I pull the cap away and casually connect the hose. But, if there is any pressure, then I have the hose at the ready under the cap & ready to connect very quickly as I pull the cap out of the way. By being under the cap, I take advantage of the first rule of plumbing--stuff always goes downhill--and minimize the spillage.