So even though we aren't going to have a potty we still need a place for the grey water(sink) to drain to. I read about someone else on the VSTF cutting their original tank and repairing it with a 3M Fiberglass repair kit, so that's what I did.
Here is the original tank that the original toilet and sink drained to, the thing was massive and nasty. Imagine a 50 year old tank whose only job was to collect filth. But I survived, and saved myself a lot of money.
Tank cut to the new size, once again I used my Festool Trion Jigsaw, also if you do this wear a respirator, fiberglass is nasty stuff to breath and guess what else is in that tank.
I cut the end cap off the piece of the tank that I was not going to use to make up the other end. What you don't see is me scrubbing the inside and soaking the tank in a 50/50 bleach/water solution.
Tank glued and patched back together, notice the notch, this is so it fits around my built-in nightstand. I am doing a leak test in this picture, the new tank holds 10.5 gallons. I also replaced the drain on the bottom.