Vinegar and baking soda in holding tanks?

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.

My take on baking soda as a cleaner was the same as your's until I tried it on my old greasy, baked-on oven debris. As long as it is kept wetted, it's a real effective cleaner for rough cleaning jobs. I sprinkled some on my dirty oven and sprayed it with water over the course of a week or so. At the end of that, it lifted all the old greasy soil off the porcelain oven surface and left everything sparkling.

If a guy could get the concentration high enough, it would probably be effective on tank surfaces, too. I'd be tempted to use some CLR or other acid based cleaner if I thought my holding tanks needed to be "hospital clean".