Happy medium between comfort and good gas mileage to haul?

Any given vehicle uses about the same amount of fuel towing any high walled TT, 4000 pounds to 10000 pounds.

Check out the 2014 Jeep Grand Cherokee diesel and the VW Touareg TDI. They will tow many travel trailers under 5000 pounds dry and get 13-16mpg, about the best you can expect for vehicle towing. They are mid-sized SUVs with 4WD/AWD. The diesels offer over 400 lb-ft of torque in the 1500-2500rpm range, right where people want it.