E10 has nothing to do with anything. The only thing it will remove is the tarnish out of the tank if there was any. Methanol ie (gasohol) from the 1970's is what would strip into aluminum. Methanol is very corrosive. Ethanol, which is what we have now, is just grain made, much like Everclear, only stronger (199 proof) and they add drip gas to make it not for consumtion. Smells strong like tequila if you have every been around it before they add the drip gas.
I did alot of testing for the Corn Growers Association back in the 80's for racing applications as well as for the producers in the 1990's. The end result was Indy car. 100% baby
Anyways, like I said, filter it out. I really doubt Four Winns made a boat with a Plastic fuel tank, I might be wrong, but I doubt it. Anyways, unless it's in direct sunlight, the plastic they use for fuel holds up for decades. It is all they have used in many applications forever.
If you really need an answer, pull your sender out of the tank and see how much it has deteriorated. Possiblility number one.
Most likely, someone either gave you some really crap gas with additive that went south, or someone put some in.
Filtering it out is the best answer.