I had a similar issue with mine when I bought it, I suspect it had been parked for long periods with 'a little something' left in the tank since it was a one-owner, rarely saw a campground unit.
I tried a bunch of different things, what finally seemed to work was to 1/2 fill the tank with clean water from the garden hose on top of a double shot of odour eliminator / digester stuff. I left it parked that way for 2 weeks then took it for a2 hour drive to the in-laws where there's a 'dump station' (access port on septic tank).
I also found a clear 45 deg. elbow with a water jet thing built-in at CW, handiest thing ever!! It allows you to monitor flow on the line and selectively fire a jet stream of water back up into the black tank. By alternating spray, drain, spray, drain I was surprised at how much more 'stuff' came out even after the tank was supposedly empty!!!
Ted 'n' Laurie, plus Jackson (aka Deputy Dog, the Parson Russell Terrier 'fur kid') and, Rylie (who crossed the Rainbow Bridge June 14, 2012).