Laundry detergents and fabric softners both cause "crusting" in septic systems, because the two chamber systems flows liquids into thelarger chamber and holds the black stuff in the front chamber.
Laundry detergents have fillers andsurfactants in them, Fabric softners have oil in them to replace oils lost in the fabric washing process. These all combine to cause crusting. IMHO you won't have to worry about crusting in your holding tanks, because the liquids aren't held long enough to start the process.