3.5 EB needs minimum 87 octane. Here in the Rockies regular unleaded is 85 and I have to run "mid grade" 87 octane minimum. At lower altitudes, regular unleaded is 87.
I did try one tank of "Eco 88" which is 88 octane E-15. I can't remember if Ford specifically says not to use E-15 in the EB or not, I just wanted to see how it did. It did OK but MPG was a bit lower than normal at 16.7 in mostly suburban/city driving (this was after our camping season so I wasn't towing the travel trailer anymore.)
I can tell you that running lower octane fuel while towing in high winds results in abysmal MPG. Going across Kansas in September in high cross- and head-winds, I got two tanks below 7 mpg running 87 octane regular. After that I made sure to always run premium which is 91 octane in the Rockies and 93 octane at lower altitudes. Definitely runs better and uses less fuel when running premium, but the $.50 or more price premium is hard to take when you're filling a 36 gallon tank. Most often the difference is more than $0.60/gallon for premium vs. regular so figuring that I typically fuel up around the 30 gallon mark, running premium vs. regular means $18 more per tank.
I had several fuelups over $100 on our Summer camping trips this year. Highest one was around $115.
EDITED TO ADD: As much as it hurts to fill up the 36 gallon tank, one nice thing about the big tank is that if I'm careful I can plan out my fuel stops and only fuel up in the cheaper locations. When I'm planning a trip, I use Gasbuddy.com to find the cheapest gas along my route and then arrange my fuel stops so I'm not buying gas in the most expensive areas.
MOST of the time this works except when events conspire to give me unusually bad MPG, like that trip to Kansas where I burned through 30 gallons in less than 200 miles (196.) Cross winds and head winds were beating the hell out of us on that trip and our trailer has a very high profile so that made it worse.