Well I am glad that this subject came up but in my opinion there's really only "one" proven method of really cleaning holding tanks and that's through the use of a 3,000 psi Hydro-Jet.
First of all you actually have to look inside the tank to see what the issues are. By use of a closed circuit TV camera lens attached to the end of a snake the inside of my tanks were made visible on portable TV monitor. This technology was employed on both the gray and black water tanks.
What was seen would make the heartiest soul cringe but not to worry a solution was at hand.
I met Mr. Don Zimmermann at a Rally in Casa Grande, AZ and he was a vendor at this event. The company that he works for is called All-Pro Water Flow of Central Arizona. Both Don and his son came over to my rig and gave me a technology demonstration. Initially using a camera they were able to identify the problems and came up with a game plan to eliminate the odors in our tanks.
By means of a long clear sleeve that is attached in between the discharge outlet and the sewer pipe they were able to insert a 1/8" diameter high pressure hose. The hose was fitted with a proprietary head that had 7 discharge holes angled back from the tip of the head. Once inserted and the line charged the hose powers itself into the tank. By manually directing the hose the jets were forced to impinge on all the interior surfaces of my tank and were subjected to 3,000 psi of water pressure. The size of the jets we're only a few thousands of an inch across perhaps as big as a small sewing needle.
This pressure is more than enough to scour the inside of the tank, the face of the sensors, the flushing nozzle and the discharge pipes. The process took about 35 to 40 minutes of moving the line in and out and twisting the hose around. After the process was completed, and the tank completely drained we checked the monitor gauges and they were all remarkably on zero.
This process was also applied to the gray water tank and it was cleaned out as well. What was remarkable about the stuff that came out of the gray tank was that we were seeing black mold clumps that were being dislodged from inside the gray tank. Mold is one of the leading causes for airborne infections in humans.
Once the tanks were physically clean, a commercial grade disinfectant was measured and dumped into each tank and the flushing process occurred again for about 15 minutes per tank.
Concluding the disinfection of the tanks, All-Pro Water Flow states that it kills 99.9% of all bacteria, fungus, mildew, viruses and germs in the tanks.
At this point we considered our tanks to be in like new condition.
Note the special adapter pipe
Don provided me with a paper that I thought I would share with you called:
Tips For Healthy Holding Tanks.
Before Your Trip
Put 5-gallons of water in all tanks
Add a good enzyme to break down solids and deodorize tanks. (Wal*Mart has a powder product on their shelves)
During Your Trip
Keep both black and gray dump valves closed even if there is a dump available while camping.
Dump the tanks before you leave and refill with water then dump again when you arrive at the new park. The sloshing action during driving will help clean the tanks and break down solids. (No need to use ice cubes)
BEFORE dumping top off the tanks with water, it creates a better exit flow and cleans the tanks.
NEVER use a dry tank, always put in water and enzymes in first.
After Your Trip
When possible, return home with tanks half-full of fresh water to "stir" the tanks.
Top off the tanks and dump before storing.
Use a quality enzyme to break down solids.
Never store an RV with dirty tanks.
Never use acids or bleach in your tanks, they can destroy the rubber in your blade valves.
In cold country, store completely clean and dry or use anti-freeze in the tanks and traps.
Because build-up is inevitable in all the tanks, consider having the tanks Hydro-Jet cleaned once a year to keep the tanks odor free and the sensors reading correctly.
There's more information at All Pro Water Flow.com