Are the cab and bed configurations the same? (ie: 4 door long bed, cap on the bed...etc...). Aerodynamics (or lack thereof) are typically the driving force behind RV MPG...and as you found speed. Also, makes me wonder what was happening with the old F250 if the MPG was falling drastically with the same trailer.
If I was in the market for a 1/2 ton, I would seriously, consider the 3.5EB. I would just keep a close eye on the temps as there is less mass to absorb the heat when doing a long climb, so if the cooling system isn't in top condition, it's more likely to have an issue. I would have no concern with it generating enough HP.