Vinegar and baking soda in holding tanks?

Put about 10 boxes of baking soda in the tank ... then pour about 4 gallons of vinegar into it. Call me when you do this as I want to watch . . . from a distance! :^)

Baking soda and vinegar react to produce carbon dioxide and sodium acetate in solution. The carbon dioxide has no real cleaning benefit (though dissolved in water it is acidic, which may help very slightly with some mineral deposits), and sodium acetate is also so far as I know not a particularly effective cleaner, though it does have some other industrial and commercial uses.

Plain baking soda is useful as a cleaner mainly in being a mild abrasive that is soft enough to not harm many surfaces. Vinegar is useful do to its acidity mainly. Neither one is especially effective against soils and general dirt (if you aren't actively scrubbing). Laundry detergent or dishwasher detergent and a lot of water would be more effective than either, in my estimation. Scrubbing with a brush would also work very nicely, but is very rarely practical and probably never pleasant for a holding tank.

I also use the Calgon and laundry detergent method - I use the HE detergent so that it doesn't foam up.

^ 2 cups Calgon along with a 1 cup of laundry detergent. That will make your tank sparkle.

put quarter box calgon in the tanks it,ll keep the gauges clean.

Oh, RIGHT. I think I heard that before!! GADS, thanks for pointing that out about baking soda. Will skip that part.

Baking soda will neutralize the acid(vinegar), that will defeat the purpose of dissolving the hard water scale.

I think that combination mostly produces a "gee whiz" effect.

The "boiling action" is from carbon dioxide that is released from the chemical reaction and all it provides is a little agitation for the acid vinegar.
I don't think that any other useful chemical is produced by the combination.