Towing kills mileage...it's just the way it is. Here's some tips to help improve it a little bit:
Keep your speed between 55 and 60 on the open road and constant.
Add a cold air intake and a freer flowing exhaust. Better air in and more air out really helps the engine run efficiently.
Install a lower temp thermostat. A cooler running engine runs more efficiently.
Get a programmer. Bully Dog has one for the HEMI that has a tow mode that may help. They also have a couple of products that claim to maximize mileage. One of the best programmers for the HEMI is the DiabloSport programmer and I suggest you check out DiabloSport's website for more info on their unit. Edge Products also makes a unit called "Mileage Max" an inline module you plug in. They don't specify which engine it works with, though.