There is no "standard" procedure and it all depends on your own particular RV and how clean you want the tank. No need to sanitize and never use bleach, formaldehyde or other strong chemical.
You should never close the black tank valve when using the tank flusher. If you ever get distracted for some reason and forget about it, you will a disaster on your hands and a potentially serious one in some cases. If you want/need to fill the tank before dumping, do it via the toilet.
There should always be an anti-siphon/backflow preventer installed by the factory and it is normally somewhere behind the hose connection outside (looks like a city water inlet). You should be able to find it as it should be accessible in case it ever fails.
Some tanks are harder than others to dump and flush due to shape/dimensions. Some just need a dump and quick flush and you are good. Some, like ours, take a lot of effort to get clean - tank is only about 6" tall below toilet but tank is 6' long. I leave the rinser running 5-10 minutes and see if tank bottom is clean by looking down toilet. If not, I will fill the tank with some water, dump, rinse and check again. I think most connect to the rinser inlet fitting by undoing the potable water hose from the CG faucet and connect your dedicated flusher hose (should be a non-white color like grey). You can use a wye at the faucet with shutoff valves on it and if you have checked to see if you have a backflow/anti-siphon valve inside, you are good.
If you are having false readings all the time, you're not getting the tank flushed out well enough each time and slime is still stuck on the sensors. Some simply ignore tanks readings though and use ESP or wait until it won't take any more contents. The tank *might* look clean from peering down the toilet (if you have a straight pipe) but a built-in flusher installed next to the toilet won't reach the sensors with a strong enough force to do much, if anything. A backflush elbow will help somewhat but won't hit the sensors.
Sometimes, the occasional use of an RV tank cleaning product (not the normal chemical used all the time) is the only way to get a tank thoroughly cleaned. Thetford makes one. We're about to give our tank a dose of Liquid Drano which our owner's manual says to use if needed. This is the only exception to using a harsh chemical. (Never dump this stuff into an on-site septic disposal system.)
BTW, don't forget to put 3-5 gallons of water in the tank before use each time so you minimize the chances of the solid stuff from getting really stuck to the tank bottom.
