I can't tell you what we get towing, but I will post again after this weekend when we take our 183 out on her maiden voyage!
I would like to point out a few things regarding fuel mileage that apply to towing and regular driving...
-Always maintain proper air pressure in your tires! Under inflated tires can drastically reduce fuel mileage and more importantly, handling. They also cause excessive tire wear and are prone to overheating and blowout. There is no good reason not to check all of your tires on a regular basis and especially before heading out on a trip. It takes 10 minutes max. A handy tip, take all of your valve stems off and check the tire pressures before inserting coins into the maching (if you don't have one at home). It will save time and money in the long run!
-Check your air filter! Better yet, go to your local parts store and pick up a new, high-flow air filter that you can service yourself. If you do, you will see a noticable increase in fuel mileage and you may also "feel" a little extra umph as air flow naturally increases horsepower. If you buy a lifetime filter, don't throw away your old paper filter! You will need it when you service your air filter later down the road. It needs to air dry, and you will need to stick something in its place while your air filter is laying in the garage drying.
-Watch your speed! Traveling over 60mph while towing or otherwise steadily decreases mileage with each MPH over. I tow at 60mph for several reasons, including fuel mileage. Other reasons include safety, decreased "oh crap" feeling if/when you experience sway from strong winds or passing rigs, and the other important reason... wait for it... I'M GOING CAMPING! RELAX! Get there in one piece and enjoy the ride/scenery! Driving a constant speed also dramatically reduces the number of lane changes and passing of cars on the highway. This makes for a less-stressful trip and a safer one. I am not condeming anyone for driving 65 or 70 while towing. I simply don't do it for the reasons stated above. I am a former truck driver (hualing waaay too much weight in coal out of the mountains of Maryland and WV). Speed is your enemy when things get bad and 60 is a comfortable speed for our setup.
-Notice the position of the throttle. Have you ever accelerated and edged off the pedal a bit just to realize that you're still accelerating at the same rate with less pedal? Often times towing, there is no advantage to giving it all she's got... or getting close to it for that matter. Less throttle conserves fuel and will often give you the same or comparable performance.
These are just tips... not meant to fit everyone's driving style or ability... just throwing them out there.
Be safe!
Foxhunt