Towing MPG is what it is.... If you are getting around 10 mpg, then that's about all you are going to get...
No bolt on gizmo is going to make that much of a difference, if any at all. The only thing that will make a difference is how you drive the thing and changing your driving habits is free!
When on the hwy, keep it to around 60-62 mph in third gear. This will keep your rpms just under 2500. On the hills, let the trans shift to second and then move your gear shift to second. Then hold around 50 mph. You'll be pulling around 3500 rpms, but you won't be in the throttle all that much (around half way) and you'll get up the hill just fine.
Not much you can do about towing in the city... That's where you will suck the most gas. Try to get up to speed and then let off the gas to hold speed and let the thing 'coast' as much as you can when approaching the stop lights. If it's yellow, then don't try to stab the pedal to make it... Just stop....
Mitch