Cleaning Black water 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.

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

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.

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.

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

Drano has caustic chemicals in it. It may hurt the tank, seals. Don't think it it safe for septic systems which a lot of parks are on.
There are plenty of RV safe chemicals out there you can use.
Not sure why ppl are so concerned about having a squeaky clean black tank. It's just going to get dirty again when you use it.
Make sure you have a few gallons of water in it all the time.
I like to dump when I first get to the rv park. That way everything hasn't settled yet.
I am no expert but this has worked for me for years.

good luck

You guys are quick. By the time I wrote this 5 ppl had answered.

I never never use anything but water in my black tank. I do have a Flush King sprinkler like thing I run when flushing. Keep it simple. I have been doing it this way for many many years. No odors, no worries.

It's a tank to hold poo. It is not suppose to be "clean". If it is functioning correctly and you don't have odors you are ok. If you want to do something instead of putting a few gallons of water in put a few bags of ice in it and drive it.