My current RV gets around 12 mpg or sometimes a shade more (V-10). Then either the same or a bit less when towing the C-Dory (10+ mpg).
I liked the idea of 18 mpg (who doesn't!) when RV shopping, but when I did the math it didn't pencil out for me. Couple of reasons:
1) Of the RV's on the Sprinter chassis, only the ones after 2008 (as I remember it) could tow 5,000#. Rigs that new were very expensive compared to mine.
2) The service costs looked like they would be higher for the Sprinter than for my Ford chassis. Not necessarily a bad thing, but I put it in my calculator.
So for me, it would be many more than 17,000 miles of the "extra" fuel before I would even touch the cost of the Sprinter. I was actually surprised at how much "extra" fuel one could buy before things "evened out." It was a lot. The funny thing is, there IS some psychological benefit to getting great mileage and buying less fuel at the pump, even if it costs more ultimately (purchase/maintenance cost). So I have to remember that when I'm only getting 12 mpg.
But of course cost isn't the only reason to choose one RV over another, or we would all be driving 1979 Dodge Tradesman based rigs (or the like). Preferences play a big part. I just mention this since the topic came up and on first blush it would sound like the person getting 7 mpg was way behind financially.
One other factor for me was the likelihood of finding service/shops/parts for my rig in the boonies (Ford is pretty common). On the other hand, I suspect I might prefer the driving experience/ergonomics of the Sprinter and could see myself owning one someday. Right now the ones that will tow my C-Dory are a little bit "too new," and also there isn't really a layout I like for my current travel style. That last part rules it out regardless of cost (i.e. I wouldn't always buy the rig that penciled out the cheapest if that wasn't what I also wanted).
Choosing RV's.... you can talk about it nearly as long as you can about choosing a boat (maybe not quite as fun though).