E350 tow hitch

I'd suggest going to a metal fabrication shop. I had a 24' Class C on an E350 which came from Coachmen, with a 3,500# rated hitch, mounted on a frame extension, which was considerably longer than yours. I pulled off the existing hitch and the metal shop fabricated a hitch which they bolted into the main frame. It was a rather long, strong, beefy thing. Worked fine. It was cheaper than any "normal" hitch I could have bought at the time, with the bonus feature that I was not depending on the quality of the frame extensions.
I was just wondering when you wrote deck boat, are you talking about a relatively light pontoon boat, or a considerably heavier glass deck boat? That would definitely make a difference in what you need for a hitch.