as I sometimes use the 3rd row seats and don't want to swap these in and out when going to or coming from a trip, I was thinking about another solution for specifically a watertank, by utilizing the space of the 3rd row footwell and under the 2nd row seats. would be a custom GRP solution I reckon. will fiddle about it this year, making the shape out of laminated insulating boards.
removed 3rd row backrests would nevertheless give plenty of space for tools.