Two cups of bleach may be appropriate to really shock your tank and system but only if you have a full 123 gallon black tank.
The formula for bleach use, for a troubled system, is "bleach in oz = 0.13 X tank size". And for a normal preventative it's 1 oz bleach/60 gallons.
Too much bleach is unnecessary and does damage to good things in the system not to mention could conceivably cause issues at the local septic tank or sewage system when you dump. Kills good bacteria and damages seals. Info from a RV full time chemist who retired from the job of maintaining a city's sewerage system.
Just enough bleach is...just enough. Just sayin'.