Towing MPG

Nope, that's about average. It's more about frontal area then weight. The R-pod isn't nearly as aerodynamic as we think..

It takes X horsepower to push the wind out of the way of the pod. It takes X amount of fuel and air to make that X horsepower. So, pretty much regardless of what you get mileage wise when not towing, every one get's about the same when towing. +/- a few percentage points driven by terrain and weather.

Two things you can do to help your mileage:
1. Limit your towing speed to 60 mph. The faster you travel, the more air resistance you have. You may also find that in some states, the legal towing speed is 55 or 60 mph anyway.
2. Get an air deflector for the top of your Frontier. If you do not have a roof rack, look at the Icon AeroShield. If you do, look at the Purpleline AeroPlus. I have the AeroPlus. Either of them will help to direct the air up and over the top of the 'Pod instead of hitting the front directly. A side benefit is that you won't have to clean so many bugs off the front of the 'Pod when you get home.
3. Limit your engine RPMs to 3,000 or below if possible. On the uphill climbs, this can really help with the mileage. You won't get up the climbs as quickly, but your engine won't be sucking down so much gasoline. You may be able to use cruise control in the flatlands, but in the mountains, cruise control is the enemy of good mileage. You may also find that switching the overdrive off in the mountains will help as the transmission will not be shifting in and out of overdrive which is stressful on the transmission.
You can see the AeroPlus and the way I mounted it in my mods (link in signature).
Between the two of these, I get 11-14 mpg, depending on terrain. Ours is a 2010 Nissan Frontier LE Crew Cab 4x4 with 4L V6 engine.

Even more than the frontal area is the "vacuum" that burbles behind the R-pod as you trundle down the road. For those of us with the flat backend (like the 179, 180, and a few others), there is a very large area behind that kind of expands and contracts as you go down the road.

I've been looking at vortex generators, which will disrupt the bubble, and reduce the drag introduced by the big flat backend. The spoiler at the top-rear of these models does this to some extent, but I'm not sure if it fills the entire void.

I've looked at these vortex generators: http://www.airtab.com/index.htm

But they are rather large, and I would have trouble fitting them in the available space in a few places. They have a picture of a mini-drop with the tabs installed in this web page: http://www.airtab.com/application-rv.htm

We get closer to 14 MPG when pulling our R-179, but we are limited to 55 MPH here in California.

More power will help in the mountains, but you may not experience as much of an MPG gain if you will then be trying to tow faster. Personally, I am waiting for the Ford F150 Diesel or the Nissan Frontier Diesel to be available.

If you're going to tow your pod, you are just going to have to get used to lousy mileage.  And the faster you go, the worse it will be. Stephen's suggestion of trying to keep your rpm under 3,000 is probably the best thing you can do when climbing. Not exceed the speed limit for the trailer tires (most at 60) will also help with the mileage.