What chemicals to use

It takes time, but fill your tank and then add a a box of Rid-x, used for septic tanks. Rid-x is a yeast (an emzine) that's made to break down solids. The only problem is, it takes time to do it. So fill the tank and let it sit a week or so. The build up will begin to break down and then you'll be able to drain it out. You might have to repeat a few times to get it all out.

You can also use an emzine eater for keeping drain pipes clean. In this case, I don't think you want a full tank of water, but enough to make the build up wet. The emzines will eat and dissolve the solids. Once soft, the solids will flush out.

Bleach is OK for sanitizing, but you really want to flush it out and not sit. It will deteriorate your slide valves and rubber if left alone.

Oasisbob wrote:
Maybe some hot soapy water and a drive. Really would not reccomend much more than that as it will be hard on seals.

Since OP mentioned bleach, I assume you're talking about the effect of bleach on RV "seals".

The only seals or gaskets in RV holding tanks are the blade valves and the toilet gasket. The blade valves probably use Santoprene which is unaffected by bleach. While a toilet gasket may be adversely affected by bleach, just avoid pouring bleach onto it or immediately flush the bleach off with water.

Bleach will do next to nothing to break up solids and scum in the tank. It's an oxidizer and disinfectant, but not really a cleaner in the traditional sense.

Dishwasher detergent (not hand dish soap) would probably be the first thing I would try. Leave it in and drive around for a good while, preferably over curvy, bumpy, or hilly roads to get good agitation.

Maybe some hot soapy water and a drive. Really would not reccomend much more than that as it will be hard on seals. Also would reccomend a permanant sprinkler type flush kit be installed ASAP. Can you see anything through the toilot?

I would fill the tank with at least 5 gallons of water, wait for a day to let it soak in and then take a drive on a bumpy and curvy road several times. Some folks dump in a couple of bags of ice cubes to help break up stuff as it sloshes around.

Dump to see if that helps... a clear hose attachment helps to see what comes out. You can get one online or at an RV supply store.

An alternative (and will take longer) would be to fill the tank nearly full then dump in some Thetford Waste digester chemical. I prefer the powdered kind. Be sure to use the amount suggested based upon the size of your holding tank and maybe one extra envelope for good measure. Take a drive to slosh it around and then let it sit for a few days. Take another drive and then dump.

Hit the bu