Bubble Top Van vs Roadtrek Style Gas Mileage

Great question. I was hoping for an 18 mpg target for a Class B before buying. Most affordable B I found was what I bought, a 1996 Coachmen with that distinctive over-cab bed. Chevy V-8. I love that style for storage and pivoting area inside, but ours is a tight fit for sleeping. Anyway, I am struggling to get 14 mpg. Looks like 13 is about what I get so far, and with a/c use, it can be under 13. But just had a major tune up to pass a failed emissions inspection, so now I hope to somehow get to 14 mpg or higher. Will need a few months to know. I know I have a lead foot at times, I may try some 55 mph driving to see how that helps.