I've read posts about water softener slickening up the insides of the tanks on various RV forums quite frequently. One guy got the idea to put Rain-X in his black tank to accomplish this. He says it works. I don't know. Other people have tried vegtable oils, mineral oil, fabric softener, etc. Water softener does NOT make the insides of the tanks slick. Instead, the water softener is treating the water; not the tanks.
Water softener softens water. It doesn't make the insides of the tanks slick, it alters the water to keep stuff from sticking to the tank's insides, and combined with detergent will cause gunk stuck to the insides of the tanks to let go. Another way of looking at it is that water softener, detergent, and chlorine bleach, are all working to CLEAN the tanks. Adding vegtable oil makes the tanks dirty.
I also see posts that warn that chlorine bleach will make plastic waste water tanks brittle. I love that one. Those who make that claim seem to have never noticed that chlorine bleach is sold in PLASTIC jugs.
The idea behind The Geo Method is to use inexpensive, yet very effective, laundry products in NORMAL combination to CLEAN AND SANITIZE waste water tanks.