The comments here amaze me.
Heavy? This thing is no heavier than any wooden platform that you guys recommend here ALL THE TIME. Couple of sheets of plywood and a few 2x4's.
If it leaks, SO WHAT? It's just going to leak into the truck bed and run out the back or the holes in the front corners. The camper is high and dry. You shut the factory gray discharge, pull the tank, and fix it.
What tanks are readily available? Is there a 4'x8'x4" high plastic tank on the market that can withstand the weight of the camper? What is your idea? Where else on the camper are you going to put more gray capacity?
Frankly I think they could have much more easily solved the problem with compromise, by not taking 20 minute showers, but that's just me. A long luxurious camper shower for me takes 2 gallons of water.