It’s an EBay special. Made of really thick plastic. I’m not sure what its original purpose is. The best guess I’ve heard so far is a school bus fuel tank.
But I really don’t know.
Its not totally plumbed yet, but I’m going to set it up to refill my rear tank. I’m hoping to be able to flip a switch in the cab that will open a solenoid and fill the rear tank while I motor down the road. :Sly