Cleaning Black water tank

Do NOT use Drano. Use Calgon water softener, dish detergent, and water.

No, I've not ever heard of putting Drano in an rv tank, and wouldn't do it in mine. There are other ways of cleaning/maintaining the tank which are not so harsh. Most important is to not skimp on water. The black tank is not a place to conserve water, and you should ALWAYS have a minimum of a couple gallons in it during season, even after dumping.

Better to put in a capful of Calgon Water Softener and a 1/4 cup or so of Dawn dishwashing liquid before driving around.
Even with that the sensors may not work all/some/most of the time.

If you really want to do something, search online for "GEO" method. It will instruct you in the safe way to clean out the black tank

That is not normal and likely not safe for the seals in the drainage system.
You really need to do nothing more than keep plenty of water in the system. Put a couple of gallons of water in the tank before you begin your travels. Do not dump the tank until it is at least 1/2 full (3/4 is better) and when the black is empty, open the grey tank and let that empty and clean the dump hose as well. Close it all up (I recommend you never leave black or grey valves open even when parked) and start the cycle over again when it fills up again.