Black tank chemicals come in several flavors
Formaldehyde based: I'm told (rumor) they are illegal in some areas/states. Since I've not visited those areas I have to say "Rumor" (well have visited but not with RV and don't use 'em)
You wnat to PRESERVE the stuff for burial after death.. I think not
Odorants/De-odorants Generally not worth the money>
Biologicals... THey may work but I've yet to see evidence of it....
WATER.. Works well. Low cost
now some "hints"
The sensors work.. when brand new and likely never again (Will give false higher than actual level readings)
You can do a fairly good job of cleaning by adding about 1/3 tank water, 1 cup each DAWN dishwashing liquid (DO NOT use the anti-bacterial) an Calgon water softener compound (Liquid or powder) and Driving for an hour or so then dump again. Tank may show full (of bubbles) after dumping but may show empty (or not) a couple days later.
But be advise I make no guarantees and the phrase "May not work" applies.
I would only do this end of season.