I bought some enzyme stuff today. I left the valve closed for one shower, and then went and opened it to see the flow out of the tank. The flow out of the tank was only a trickle so the problem is more widespread (black tank comes out full bore.) I am chemically sensitive so have to be very careful what I expose myself to in terms of vapors; so after tonight's shower gets done draining as much as it can, I'm going to close the valve and put this quart of stuff down there and let it sit overnight, to see if it eats through the clog. If that doesn't work I'll try more forceful means. Thanks for the suggestions.