+1 Ronco Plastics
Ronco Plastics marine Tanks They make TOP quality thick-walled water and waste tanks for a very reasonable price and has more than 400 shapes and sizes, over 100 of which are non-rectangular (which means you might be able to fit a larger one in the same space), and they install fittings in the sizes and locations specified by the customer when they make the tank.
Duraweld
DURAWELD supplied most of Hunter's water and waste tanks in the '90s (and may still), so you may be able to get the dimensions of your tank from them. However, as you'll see on their website, their standard wall thickness is only 1/4" compared to Ronco's standard 3/8" minimum...You can request 3/8" but for a considerable upcharge.