There is an excellent sale price at Camping World right now on Walex Porta-Pak toilet chemicals for $5.63. I bought these in Columbia, MO but I would think the price is nationwide, I could be wrong though. Walmart sells these at just under $10.00 for the same size.
I know a lot of RVers use the Geo method but I prefer to use toilet chemicals that help break down the solids in the black tank.
BTW, it is incredibly windy today in Columbia as you can tell by this picture of the huge flag at Camping World completely unfurled.