Your main MPG factor is how you drive and how fast you drive. Your second most important factor is the aerodynamics of the vehicle. Most any TT will be about 8' wide and 9' tall. Any time you're basically pushing a "wall" that size thru the air it will use a lot of fuel.
Once you're cruising along weight doesn't have much to do with it. I don't think any change in trailers would make any significant difference in MPG, certainly not enough to justify getting rid of a trailer I presume you like.
An Airstream trailer might get you some improvement but they are rather expensive so the trade still wouldn't be worth it financially.
Really, 7 MPG is pretty good for that size of a rig. I think your best bet is a bigger fuel tank from Titan. Or, as you already know, a diesel would probably get nearly double the fuel mileage.