I wanted to replace the water tank in our '79
Trillium 4500 and sent the dimensions of the current tank to a place I found on line that will make any size water tank with inlets where you want them- They quoted me over $800 for a custom tank so I did a rigorous sanitation and put on a water filter. A long way of letting you know, from my experience, that if you can you should get a "standard " sized tank and maybe mount it in one of the under seat storage areas or be prepared to open your wallet wide. A standard 15 gallon or so capacity tank can be found from an online rv parts store for around a hundred dollars. YMMV