Firstly, your issue is with the Landowner, not the farmer who is renting the land. You should address your concerns to the Landowner.
If the tank is 80 years old, it would probably need replacing anyway. It will be an old brick lined system with two tanks, very unreliable and prone to flooding, blocking and spewing out contaminated sewage. They also need soak-away, usually a herringbone construction that can get damaged by heavy farm machinery.
Even if your parents have the right to dispose of waste into the tank, the legal fees to assert this would far outweigh the amount of money to change the system to what The Builder described.
Also, should the house every have to be sold, the fact the septic tank is on someone else's land will impact negatively on the Sale, especially if there has been disputes over it.