Is this different from the stock Ford E350 fuel tank? Typically I thought motorhome builders used the factory tanks, and a very quick search suggests that replacement OEM-style tanks are very readily available from the usual places such as JC Whitney. If you search for Winnebago, you probably wouldn't find them, assuming it's a normal Ford part.
One of the nice advantages of E series chassis is that Ford has made about seventy bazillion of them over the years so parts are generally not difficult to come by.