I had my gray tank leak only when completely full. The pipe going into the tank wasn't sealed good. I put a hose clamp on and got it tightened the best I could. I also bought some of that "flex seal" stuff. It wasn't the brand seen on TV but a brand from the local hardware store. Hasnt leaked since. Mine is a MH in a heated compartment. And it would be a huge PIA to take the tank out. So I tried this first.
It had probably been loose since new, but the gray tank had probably never been that full before.