Alpine Buck, there is a slim chance that the septic system regulatory agency in your area has a drawing of the septic system at your home.
If not, you may want to call the local Septic Tank Pumpers to inquire of them if they had ever pumped your specific tank, and if they can remember the location of the tank.
If no luck there, ask one of them to come to your home to pump the sludge from your tank, and inspect the tank to be sure that the baffles are in place, and fully functional. Septic Tank Pumpers find these hidden treasures on a daily basis, and most of them are practiced in this art of locating septic tanks.
Be sure to tell the selected Pumper that you do not know the location of your tank, so that he can schedule additional time to accomplish this task. It may take a considerable amount of time to excavate the soil above the access hole.
If you find the tank on your own, then excavate the soil above the inlet and outlet access holes. Digging the soil over the access holes yourself can save you a consideralbe amount of money.
Call the Septic Tank Pumper to remove the sludge, and inspect the baffles in the tank. Accomplish any needed repairs. I recommend that you install a Septic Tank Effluent Filter in the outlet tee of the tank. If your tank does not have a 4 inch PVC outlet tee, or the tee is inaccessable, then a new outlet structure will have to be installed.
Remove the hole covers (normally a concrete plug), if that is what you have, throw them away, and install 20 inch diameter risers from the access holes to the ground surface. Cover the risers with gas-tight, lockable covers. These easily accessable risers will allow for access to pump sludge in the future, and will allow for annual access to hose off the outlet filter. JWA