Hello: Polyurethane plastic tanks can be repaired if your is leaking. A fresh water tank is pretty straight forward with a fill port, and drain port, and a suction port for the pump located where you need them. Otherwise a rectangular tank of the width, length and height you have room for will do the job.
We reused our 76 Sovereign
fresh water tank after cleaning it. I kinda wish we hadn't. The tank is poorly designed in my view. The pump suction line is routed to that center square area. That is a long draw for the pump. The drain port is a plastic pipe thread which almost always end up leaking. Same with the fill port.
Inca Plastics in California is a rotomolder with a big catalog of tanks. I looked in their Airstream catalog but did not see the Sovereign
fresh water tanks. Inca Plasitics can spin weld fittings where you say you want them. I use their rubber grommets which makes the job a bit easier.