how will ethanol affect my fuel mileage ?

I am no expert but here are my thoughts. Ethanol has less BTU output than gasoline, thus you have to feed it more. Right off this tells me that it's going to cost more to drive the same distance. The other issues are to have the proper seals in the system to cope with the ethanol. Now if there was a significant price difference between E85 and gasoline (more than what it is around my location) you might would break even on cost. E85 can have as little as 73 percent of the BTU output of gasoline. Here's a link.
[ame]http://www.afdc.energy.gov/fuels/fuel_comparison_chart.pdf[/ame]
Of course I base this on the assumption your trying to save money on fuel cost. Please don't take it as I'm busting your chops. Just trying to give you some information that is valid to help with your decision.
I do agree with Mopar Daddy that if an engine is designed for it it would be more efficient. But unless your going to set the truck up to maximize its use I see no benefit or savings.