I determined that my black tank never really fully empties. Our last trip was without a sewer hookup as none were available, so I wasn't able to flush the system at the site. I had to dump at the campground. At home while cleaning I noticed it was still pretty smelly, and a look down the hole confirmed the reason for that. I filled two gallon water jugs and headed to a Pilot and dumped there. Must have been a hang-up, as the extra dump seemed to help. In the future I will always load some water jugs first if I need to dump and won't hooked to sewer.