I'm thinking that the installer would be Spartan , and perhaps the reason that the price is so high is that Fleetwood , is getting the tank from Spartan and marking it up .
Have you tried sourcing the tank from Spartan.
You could call them with your serial number.
Spartan Motor Home Support.
If they can give you a part number then maybe ; Dorman , a plastic tank manufacturer could match one up.
I just replaced my Freightliner surge tank with a Dorman product , with only two reducers in the vent lines to adapt the tank.
I'll see if I can find a web site.