With the experience of a 30 day, 4500 mile trip in our Sprinter RV and several years towing a camper, I'll humbly offer a couple suggestions. First driving continuously over 70 is really pushing your limits. Making those speeds towing a trailer is way over the edge.... IMHO. With several years of towing a 17' hybrid behind my Sienna van (with sway and equalizer bars) it is clear that getting and staying over 65 is REAL WORK and in a short time your concentration begins to fail. Its hard on the equipment and it just sucks fuel. Also, considering my analog speedo reads 2-3 mph SLOW and it's likely yours does too, you are going faster than indicated. A more prudent speed is probably 60-65 for safety, wear and tear, tire reliablity & fuel economy ...not to mention the comfort of your passengers.
Our Ventura got between 18.5 and 21.5 under various conditions but keeping the speed down to 63 (100Km/hr) with an occasional burst to 70 for passing got the best MPG. Hills obviously take more fuel and they eat into your economy. The next thing is winds.... if you are making 70 and have a 15 mph head the van thinks its doing 85 and your fuel bill will reflect that fact. Direct or quartering headwinds eat gas! No ifs ands or buts.. So if you recognize a headwind and or cross wind... slow down 5 mph. Same with rough roads.... they eat fuel much more than a smooth flat macadam surface. Again, slow down to a comfortable speed. Mine varies depending on winds & road surfaces ... slowing down or speeding up as appropriate.
Adding a trailer.... it should be no surprise that your economy goes down.
I use the CC almost all the time. Set and forget! At a reasonable speed 60 - 65 mph. I use my GPS' speed function and set my speed by the GPS as the speedo reads slower than actual.
The biggest contributor to aero drag (and low fuel economy) on Sprinters is the formation of a low pressure area behind the moving vehicle. 'Fast backs, Kamm backs are much better at reducing or fooling the air into thinking the vehicle is tapered at the back to reduce this low pressure area. Bread boxs (AKA Sprinter, box van, semi van) are bad news especially when they have 90 degree edges on the back and along the top edge. Air-Tabs are best when used on 'boxes' that have square corners and edges. Take note that Benz has carefully radius-ed the rear corners (nice gentle curves), the top sides and rear top edge of the Sprinter. This is no accident or stylist's whimsy. It is aerodynamically beneficial. So I don't think it would help much to Air-Tab a Sprinter.
After 'reading' the dirt patterns on our Sprinter, I don't believe Air-Tabs (a wonderful product I think) will do much good. I'll point to the fact that the rear windows remain pretty clean, indicating that there's not a much low pressure forming behind the van that would suck dirt to the windows.
Now, there is a collection of dirt and rain water run off about 3' up from the bottom sides. Above that the vehicle is very clean. Its as if you had drawn a line horizontally along the side below which its dirty and above its much cleaner. If you were to use Air-Tabs, I'd think you would want to use them somewhere in, and below, the dirt line. However, this area is very 'dirty' air flow wise due to the wheel openings, mud flaps, etc., that produce a lot of turbulence themselves. My point is that Air Tab might not be as effective in the 'dirty' air as it would in a laminar flow higher up (where you don't need it).
However, I am considering using AirTabs in specific areas. Several could be placed on the top rear edge of the air conditioner unit which sits like a big marshmallow on the van's roof.... along with the other warts and bumps. Here they would catch clean air and create a nice vortex off the back of the A/C unit that would (theoretically) cascade into the Sprinter's wake making it appear 'longer and sloped' to the air flow. Also, one could be placed on the rear side of the box holding the awning. Another could be placed on the 'wart' covering the refrigerator unit's chimney. The latter two would create vortex that would again help make the vehicle appear longer and sloped to the passing air.... or at least contribute to the process.
Cheers!
