Re: Islander 28 waste tank
The conventional waste tank for an I28 is a rigid poly tank, installed under the staboard settee just aft of the head. And then the fresh water tank is installed just aft of the waste tank.
I don't know the profiles or numbers, but would expect there are some choices in terms of how long you want each tank to be vs the total length available for the two.
It may be more accurate to make up a cardboard dummy for the best/max fit, and then take measurements off the dummy and try them on Ronco or another tank supplier. You'll have to remove the entire plywood "base" that covers that area to really get good access, and while you are in there, you might want to add a PVC pipe or other conduit in case you want to run wires through that space, under or behind the tanks. (Which is a convenient way to get them from the mast & head aft to the panel.)