Hi,
I just thought I'd share my experience replacing the holding tank on our IP350. We chose not to cut out the sole to install a tank where the old one was. I left the old, leaking, aluminum tank in place and filled it with expanding foam. I had a new tank fabricated to fit directly behind the toilet, against the hull. The new tank is much smaller (about 14 gallons), but has the huge advantage of being serviceable, visible, and, very importantly gravity feed. That's right, no more pumping to empty it.
All waste goes into the tank from the head (no Y-valve). You open a single valve at the bottom of the tank and the overboard discharge thru-hull and the waste flows easily out the bottom of the tank. There is a Y-valve between the tank and the overboard to select to pump out through the deck. This is how many French boats are built.
You could likely increase the capacity to 20 gallons, as I didn't use all the available space. I made a video of our experience that you can watch at:
photos.app.goo.gl/LF9GJ8vC8cDPCDVFA 
