I've been in on the installation, repair etc. of a couple hundred septic systems. However, all in Montana. Soils different, installation was probably different, etc. If you can find a clean-out or something under the house where the system exits the foundation you should be able to find your tank pretty easy. If you do have a cleanout, open it up, and you should be able to run a snake out from there. Take it easy pushing it out, and when you hit resistance, you've probably found where the outlet drops into the tank on a Tee. Pull out the snake and measure how much you had in the line. That's how far the tank from the house is. (Again, it depends on installation trends at the time, codes, etc) We always used either a 10' or 20' piece of pipe from the house out to the tank. Just made it easier later on to find them. Then a line will go out from there and the comments about different green grass, etc are a good clue. If it's old, you may even be able to smell a little if you get close to the ground. If there are any water witchers around, they can usually find a metal tank. If it's an old system, be very careful when you start digging for a tank. I've worked on systems that were installed with steel tanks and steel tops wayyy before concrete tanks. DAMHIKT, but you can step through a rusted lid. Yep, the smell stays on you for a lonnnnnnng time.
In regards to adding stuff to systems, I'll say that everyone is corect. Add stuff or don't add stuff. I've worked on systems that had been in the ground 44 years, never had a thing added, never had a tank pumped. If it's in good soil that percs well, you're conscious of what you put in the system, the bacteria will do their job. Some times we put a system in and adding a little "starter" in a year or two really helped. It depends. There's a lot of variables.
Good luck!!
Tom Swortzel