... What is the effect on the septic tank when dumping a 45 gallon black tank all at once? ...
I will relate a story about an RVing friend of mine from several decades ago.
"Jim" and his family (wife + 2 teenagers) lived on a "hobby farm" with a septic tank/drain field. And, Jim had installed a clean-out handy to the spot where he parked his 5er. (Jim held a 9 to 5 job in the city.) They had been gone a week fishing. I'm not sure if they dumped while they were away, but I know that they hadn't dumped for several days before they returned home. I have a pretty strong feeling that the holding tanks were pretty full.
They pulled in late that Sunday night. The wife immediately carried several arm loads of dirty dishes into the kitchen and cranked up the automatic dishwasher. Then, she apparently started a laundry. In the meantime both teenagers took showers. And, while all this was going on Jim emptied his tanks into their septic system. And, as they went to bed, apparently both Jim and wife also took showers.
Do you see where I'm going with this?
When they got up the next morning they were greeted with a small lake of raw sewage in their yard. And, that's when they called one of junglejim0745's compadres to clean up the mess. They NEVER
did that again!
Just food for thought.