Black water tank cleaning

I would not leave bleach in it because of the damage it may do to the rubber seal at the dump valve.

I use Febreeze softener. It's cheap and smells wonderful. It's a surfactant just like soap or Calgon. Use with plenty of water.

Downtheroad has it correct , you ain't storing food in it . My sister scrubs her tanks over and over and over , all the while her husband is pacing back and forth waiting for her to get done so he can take a dump , if I am allowed to say that !

Fill it with water for certain but bleach won't do anything to help anything other than to dry out seals. The main thing when a trailer will be unused for a period of time is to NEVER allow the black tank to dry out. Regardless of how well you clean the tank there are ALWAYS remaining solids which will turn to stone if allowed to dry and over time can form a clog.
Good luck / Skip

I swear by the "GEO" method:
One cup Calgon water softener ( or 20 Mule Team Borax)
One cup cheap laundry detergent - I use powder because it's easier to store
Mix in 3-5 gallon water (we use 3 gallon mop bucket because it's handy)dump down toilet and a batch in bathtub .
Leave in tanks till full enough to dump, rinse, repeat and leave in tanks .
We premeasure and keep in zip lock bags in bathroom.
Senders work fine-every time.

Why are you worrying about sanitizing a waste tank?

No Bleach needed.......the rinse with the wand is good enough.

Then add a couple of gallons of water back into black tank so the bottom stays wet.
Don't want it to dry out cause any stuff will dry out and stick....then it will accumulate and then you will have problems

Dump, rinse, add water......done until next time it is full. Then repeat.

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A good rinse now and then is good.
Don't use bleach...It might kill some germs, but you aren't storing your food in it.

Leaving bleach in may harm the drain seal.

Bleach won't likely help.

Here's an afternoon's reading:
dirty sensor 1
dirty sensor 2
dirty sensor 3

Clean sensors 2010
False black water reading

Yes, but make sure you rinse the bleach. Don't leave it sit in the tank, it will erode your slide valve. Rinse and drain. Then add a couple gallons of water to keep the slide seal from drying out.

It would be better if you added the bleach, about 5 - 10 gallons of water, took the camper on the road, let it slosh around real good, then bring it home, drain, rinse, and then add a couple gallons of water while it sits. This way the bleach will slosh and get on everything. Rinse well with your wand.