You can order custom (or stock) tanks from plastic mart online ......the tanks are fabricated by Ronco in Tustin, CA. They ship or you can pick them up if you are in So Cal.
I have one.... you specify the stock tank P/N (or custom dimensions) and provide a sketch of where and what size female NPT fittings you want and pay your $$.
Reach out to member 1der here....I think he is set up with this vendor to provide tanks at a bit of a discount.....it's been a while.
https://www.plastic-mart.com/category/33/rv-water-tanks