Afew years ago we converted a Merc 709 panel van and had 2 shaped bespoke fresh water tanks and a large gray tank made shaped to fit inside the chassis. They were very successful. Cannot remember the company but will see if i can find the invoice and post the details.
PS The tanks were bought via O,leary's of Beverly. Phone - o1482 868632. They arranged the manufacture based on the drawing we provided. We do not know who the manufacturer was. Hope this helps.