Retired VSP wrote:
... My wife and I had even cleaned prior to the service and allowed both black and grey leavers to remain open letting the water drain as we used it.
Any thoughts on this...
Not sure how long you had the valves open but your black tank almost certainly was retaining water since there would be TP blocking the outlet pipe. Combine that with driving and sloshing things around and that could explain everything. I'll assume that you had all valves closed while you were driving since that's the law.