When we had our 210WBS, I towed with a Hemi Jeep Commander. We averaged 10.7 mpg [2.64 miles per liter] (averaged over 48,000 miles) with the lowest being 8.5 [2.24 miles per liter] (35 mph headwind going over the Rockies into Oregon) and 12.8 [3.38 miles per liter] (15 mph tailwind crossing Kansas). You will find, at least from all the things that I have read, that the real world average for most folks towing trailers is going to be in the 10-11 mpg range. All trailers have a "frontal area" air resistance hurdle to get over and that frontal area is fairly constant for travel trailers (~8' wide x ~11' high) and that, more than weight, is what affects your mpg (or mpl for those north of the border
A good example; the rig I am talking about in the above, Jeep Hemi Commander towing not quite 6,000 lbs of FunFinder 210WBS averaged 10.7 mpg. My current rig; 9700 lbs of truck with a 6.7 Cummins Turbo Diesel towing a little over 11,000 lbs of 5th wheel gets 9.2 mpg so far and the diesel will get better mileage after it is broken in (around 90,000 miles). The loss of 1.5 mpg can mostly be attributed to the fact that the 5th wheel is 4" shy of 13' in height (as opposed to 11') and not the fact that the truck is 3700 lbs heavier than the Jeep or the trailer, itself, is almost twice as heavy. Once you get a mass rolling, it doesn't take a lot to keep
it rolling. What you do have to constantly struggle against is air resistance. The more frontal area you have, the more fuel you will consume to overcome the air you are pushing through. A pop up will get you the best mileage, a motor home, the worst. There are exceptions (there always are), but, the above is a good rule of thumb when contemplating what fuel mileage you may get.
As an aside... A lot of folks assume that a small engine will get better mileage, while the bigger engines will get worse. Again, in towing it is air resistance that consumes the fuel. While a small engine, inherently gets better mileage, it has to work harder (read higher engine speeds) to overcome that resistance than a larger engine (like my diesel) that "loafs" along at 1700 rpms ...higher engine speeds require more fuel, lower engine speeds require less so that the actual fuel used is going to be about the same. You can have too little engine (not enough engine to overcome air resistance), but, as the saying goes, it is really hard to have too much engine. Wear and tear also enters into engine selection; smaller engines, though rated for a given trailer weight will work harder and will wear faster than a larger engine that is loafing most of the time. Again, the diesel is a good example. My red line is half of a gas engine's and it does its best work from 1500 to 2200 rpms. A diesel with 100,000 miles on it is just barely broken in. Most folks are overjoyed if they can find a used diesel with 85,000 miles on it...it is almost broken in and will start giving its best fuel mileage!