Depending on your procedure for draining the tank, you could have had some "debris" in the valve gate and it prevented it from closing all the way.
The issue now is that if all the liquids drained out, it left all the solids in there to harden in your tank. If you are at a full hookup site, I would suggest aggressively flushing the black tank to ensure that you have it as clean as can be and while it is flushing, work that valve in and out gently, not fully closing it to avoid jamming the valve guides with "debris". Once you think it is good and clear, close the valve all the way and have someone run water in the toilet or operate the tank flush until you have a good amount of water in there and see if the valve is holding.
After watching my brother get a brown shower when he removed the drain cap on his one day, I decided to add a Valterra termination valve on the sewer outlet so that even if the tank valve(s) leaked, the valve would hold everything in. It also doubles as an equalizing valve so that I can open both grey tank valves and let them "share" capacity between the bathroom and galley grey tanks since the kitchen fills 10x faster than the bathroom grey.