If your grass is greener and the ground is moist, I’d say you certainly may have a problem. After 40-50 yrs. I would think there’d not be much chance of a leech field operating properly. The sink holes are especially worrisome. The presence of the holes indicate ground settlement and improper operation of the system. You should check with your local Health Dept. Under their codes you may be required not only to pump the tank, but to also remove the top and fill the tank void with specific materials.