The fact that both grey tanks leak when full seems really strange as they are plumbed independently.
I did have a similar issue with my fresh water tanks--I have two in my Voltage, where they both leaked when full because the manufacturer crushed them with angle iron when installing and then of course covered their "work" by putting the coroplast along the bottom of the trailer so you cannot see the leaks. It took a few years for plastic to finally fail. I used a silicone plastic patch and coated them the best I could. One of the two still leaks a little when full but at least it is fresh water and I saved on removing and replacing both tanks with new ones.
So I guess it is possible they both failed at the same time for you but seems unlikely.
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2018 Voltage 3305
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