Since you have a Ford F53 chassis - a Ford Truck/Commercial Truck repair center should be able to source and install a new gas tank.
I know in automobiles people do remove tanks and fill them with water and braise repair tanks. And sometimes people line in the insides with plastic. But I'd guess it's better to replace it.
You could get a used gas tank from an RV graveyard. But it could be rusty from sitting too.
2017 Winnebago Adventurer 37F
2016 Lincoln MKX Toad