Unless yours is a VERY old system, you likely have a septic tank with at least 1,000 gallons of sewage sitting in it right now. That 50 gallons of black tank with 5-6 oz of CampChem in it is NOT going to sterilize the septic tank.
We have an old saying in civil engineering: Dilution is the solution to pollution. Diluting the formaldehyde that much puts it below the toxic level for the anaerobic bacteria that chew on your sewage in the septic tank.
Go ahead and use your home system as follows:
1. Don't use massive doses of chem in your RV tank. Follow the directions for your tank size.
2. Dump a max of 50 gallons at a time unless you know you've got a septic tank larger than 1,500 gallons.
3. Never dump into the inspection hatch of the septic tank. You'll churn everything up and nasty greasy gunk will get discharged to the leach field (expensive!).
4. If you add a cleanout to avoid #3 above, make sure it is located Upstream of the septic tank. If you install it downstream, you'll be needing a new leach field soon.
5. Listen to anything bid_time says on the subject. He obviously knows his... well,you know.