I’m in the motion of installing a new septic system and I asked the septic company what the issues would be by having fruit trees around it or anywhere near it. He responded that in all his career he has never encountered any fruit trees penetrating the septic system. He mentioned redwoods and other large deep-rooted trees close by would be an issue but he had no issues with planting fruit trees near that leach field or the tank.
He said that he had installed septic systems all over Napa in commercial orchards were trees have literally been planted above it without any issues.
That said, I won’t worry about it, but where the existing leach pits are located, I’ll build a gazeebo or similar and plant the trees around it.