Love my Toy Hauler.....except one "small" thing.

I have 50 gallon black and 50 gallon grey in my trailer. 7 days at the dunes with 3 of us and we don't fill up the black. Don't know what you are doing to fill up 32 gallons that quick.
Grey will fill up over that same 7 day trip. Even with being careful. No dish washing, quick showers etc.

Not sure you will have enough room to install a larger tank. A deeper one maybe, but make sure you have supports strong enough to hold more weight.