http://www.pplmotorhomes.com/parts/rv-cleaners/83-1817.htm
If you have narrowed the problem to the Blackwater Holding Tank, I would use a chemical deoderant like AquaChem (see URL above for a source). Add more than recommended and fill the tank 3/4 full. Drive over bumpy roads to get the solution sloshing around (drive for 30 plus minutes). Stop for 30 minutes and then drive the bumpy road again for 10 minutes, fill the tank full and then dump. Fill the tank again and dump. Add a gallon of water and twice the recommended chemicals and see if that takes care of the smell.
The aqua chem has enzymes which help to digest solids into liquid. They work over time so it may take a few cleanings to get rid of any built up solids.
I add one packet of the dry AquaChem each time after I dump the blackkwater tank.