gbopp wrote:
If it's not a vent or a toilet seal problem, consider The Geo Method for your black tank.
I second the Geomethod. I just started this season and it has worked well for me. I have a seasonal site, so I am here on the weekends and away during the week (except for 4th of July vacation week!). I leave the black tank until it is absolutely full, dump it, and follow the directions on the Geomethod website.
I had to order Calgon powder from Amazon and I recently found the liquid in a Wal-Mart. I just tried that and will see how that goes.