My Syncro Westy has a Subaru SVX engine (6 cylinder 3.3 liter) in it. Back in December I started noticing a distinct drop in fuel economy and a loss in power. When I first got the camper, it had the stock engine, wheels and tires (205/70R14 on the alloy rims). I got about 18 mpg. With modifications I did (engine, larger wheels larger tires), my fuel economy dropped to 15 mpg (as displayed on the odometer, in reality about 16.5 mpg). But the added power made this an acceptable trade off, and made the vehicle much more fun to drive. 

In December, I noticed my fuel economy had dropped to 11mpg. This was seriously affecting the range of the vehicle. One of the considerations I thought about was alcohol in the gas. This affected other vehicles by a loss of about 10% (aka 1-2 mpg). However, my camper was getting much poorer fuel economy than I could attribute to the fuel alone.

So it went in to my shop for hunting down the issue. After a couple of weeks, they had found nothing. They noted the poorer fuel economy but couldn't find any reason for it.

To make a longer story somewhat shorter, it was the gas. I live in the DC metropolitan area, an area where the EPA mandates oxygenated gas to counter air pollution. Lately this has been done by 10% alcohol. However, I usually filled up my camper in the Shenandoah Valley, an area not affected by the EPA, as prices were substantially less there. However, more recently nearly all gas stations in the Shenandoah Valley now only sell 10% alcohol diluted gasoline. 

But I found a station that sells alcohol free gas (92 octane, as my SVX engine wants premium) and BINGO, my fuel economy and power are back to what I had grown used to. 

pure-gas.org also lists stations that sell alcohol free gas.