Simple! Wind resistance is the answer. 70-75 mph has much higher wind resistance than 60 mph. It isn't linear. It is more like exponential the way wind resistance increases with speed. It takes much more force to overcome wind resistance at 75 mph than at 60 mph. Add the headwind to that and you might have been fighting the equivalent of going 80 to 85 mph. No wonder that your mileage was only about 7.3. I tow an RPod 179 with a Nissan Frontier and limit my towing speed to 60 mph. On our recent trip to Florida, we were getting 11-12 mpg. We were fighting winds on the way down and it was not much better on the way back because the wind had switched, so we had a cross wind with some headwind vectoring also. Although the RPod looks like it should be quite aerodynamic, it isn't as much so as one would wish..

When we were towing with the Ford Escape, I added a PurpleLine Aero Plus wind deflector. That helped significantly. However, when we changed to the Frontier, it does not help as much because it is located too far forward to be of use. If we had a truck cap that would allow me to place it toward the rear of the cap, it would work much better, but we don't.

That seems about right, maybe a little low, which might be due to your speed and the headwind. 

TT

Terrain means everything to mileage as does your rpm's, how hard your engine is working. Keeping the rpm's down and not being the first over the hill makes a big difference. Even just driving up a mild slope on the highway and the TV down shifts to 5th, I manually kick it back to 6th gear.

When we go, we go for up 6-8 weeks and will go into Canada. Gas mileage and keeping the cost down is very important to us (especially in areas of high costs).

I average 14mpg when towing. I track this not by the digital meter, but by mileage/gallons at the pump.

Note: Check your tires for spec's. Adjust air pressure for road and temperature conditions. Check your speed rating, at 70-75mph you may also be driving over it..

MHO 

Assuming flat ground the two forces you are burning fuel to overcome are rolling resistance and air drag.  At freeway speeds rolling resistance is typically only about half of air drag when towing. Rolling resistance is effected by rig weight, tire pressure and tread, drivetrain friction, and road surface. Drag is effected by the shape of the rig and the relative winds speed (headwind plus road speed). So, since rolling resistance is already much the smaller of the two forces, trailer weight doesn't really have much impact on fuel economy. 

That's on flat ground. Going up a mountain weight obviously has a big impact, and you don't really get much of that back going downhill. But I don't know how much elevation change you had on your route. 

So as StephenH says, the best thing to do if you want better fuel economy is to slow down. Driving 60 vs 75 should, roughly speaking, increase your fuel economy by about 25% assuming flat ground and no wind, even more with a headwind. I get about 13-14 mpg on flat ground with no wind cruising at 60 in my rig.  Relax and enjoy the ride, you're camping. 

Some of us keep ir thermometers in our tool kits to check our brake temps. I find it a very useful tool and use mine to adjust my brake controller to get a good balance of brake temp between the TV and trailer brakes after a long downhill grade. 

The biggest help on hills is probably going to be trying to get as much of the energy you put in while climbing back when you go down. Going more slowly up will allow you to pick up more speed going down without having to use your brakes or engine braking which will save fuel overall (because you can coast longer) as well save your brakes. 

And yes, generally engines are more efficient when running at lower rpm because there are less friction losses from the more slowly moving internal parts so keeping in a taller gear as much as possible will help. 

So, drive like a trucker in the hills and like grandma on the flats ad you'll be amazed how much less gas you'll burn up.

Originally posted by offgrid StephenH is correct, it is air drag that's killing your fuel economy. Drag doesn't increase exponentially with speed but it does increase with the square of the speed. IOW, doubling the speed increases drag by a factor of 4. And, even a little headwind makes a huge difference. Say you had 10 mph headwind and you're going 75 mph. That's a relative wind speed of 85, and your air drag will be double what it would be driving 60 with no headwind!

Thanks. I knew exponential was not right, but I couldn't remember the correct figures. It still isn't linear though, which is correct.

+1 on not needing to be the first one up the hill though. When we were driving out west with an 80 mph speed limit, I was still driving at 60 mph. That paid off when we had our accident in Wyoming. It turns out that the legal towing speed limit in Wyoming is 60 mph, so I was not cited when we hit the ice with the crosswind. It was written up as unavoidable.

My truck seems to like 62 mph.  You never have to pass anyone and the road is always clear in front of you.  Don't try to speed up going uphill.

I'd be curious as to what the consensus is regarding driving 70-75mph while towing.  I keep it at/under 65 (less in a crosswind) based on recommendations from users on another forum.

For our maiden voyage returning from Las Vegas to home, through a nasty storm with awful headwinds all the way, we still got 14 mpg, according to the truck's computer.  Right now the computer says we're getting 24.1 mpg, but that doesn't include towing the trailer home from down south.

*[for those who tend to be literal, I'm just kidding.  I only read the Senate congressional record - it keeps me awake]