Well, I don't have a rig but I can offer up my truck's history for fuel economy unloaded for the most part. You can see my city/highway split on here using a formula to weigh what I was driving. Here's the Fuelly link to my vehicle, I'm pretty religous about tracking fuel economy:
RAM truck fuel history. Also, here's a link to look at other RAM trucks and maybe their usage can give you some insight on what to expect:
other RAM trucks.. Make sure you read their comments/notes on their rigs so you know how they're setup and if they did any towing or not.