Need B+ MPG, deciding between B & B+

I bought a Winnebago ERA last May in 2012, and I get about 16-17 mpg loaded. I think it could get 18 if I drove more conservatively. I am far from retirement, but my wife, I, and 2 kids (4 and 6) have traveled quite a bit. Most of my friends think I am crazy for driving and RVing so much, but I no longer mind driving when you have your own bathroom and sleeping quarters. My wife also drives it easily without intimidation. (I have to tell her to slow down and take it easy.) We have traveled to Florida, Colorado, and San Francisco -- all from Houston. We tend to drive and just pitstop when we are tired. Then we stay with family once we arrive at our destination. That's why I bought the Sprinter--mostly for the commute. I do think however, that when the kids get older or to college, I will end up getting a bigger RV. When I retire, it would be nice to relax in a bigger unit for a 3-4 weeks at a time and sacrifice the fuel efficiency.

-Mike