Having a modern septic tank that meets codes (has the pump out places, etc) may not be the case with whatever tank is in place there.
The old part of our house was built in the early 50's. When we moved in about 70 we did not even try to find the old tank, just knew it needed replacing. Now the rules here are different than they were in 70.
There could be a "cement" type stuff in there or dry as someone else suggested.
Try to find the tank, carefully dig around to find out what type of tank. I suspect our original tank had a thick oak top and I keep thinking sometime mowing the lawn we will fall
through. We know about where our old tank is because there was a place beyond it that had a depression in the ground where drainage fed lots of weeds.
I hope your tank is the real type and that you can get it functioning. They should be pumped out every year to remove solids. We neglected to do this for many, many years (that is why I know about "cement").
We have never used any types of additives.
I wonder if any of the neighbors would have been around years ago and have some knowledge of what type of tank and where it is located. Out here where I live I do have some knowledge of what my neighbors have.