I wanted a 210gal single water tank-that Ronco made. But the box manufacturer made a slight mistake in the basement clearance and it wouldn't fit. I took back the tank and traded for two 98gal tanks that fit much better. If you're doing either your fresh or gray water tank, try tying two together to make better use of space. Good Luck, TomC
I'm surprised nobody mentioned building a tank. I'm not confident that a welded plastic tank would stand up to 125 gallons of waste though.
I had only looked at Ronco's RV tanks which only go up to 100 gallons max. I just looked at their marine tanks and they do make larger sizes, but it looks like they would cost me just as much as the custom stainless tank.