Happy medium between comfort and good gas mileage to haul?

Diesel makes a huge difference. My ML350 with 3L TuRbodiesel had no problem pulling a Nash 25C (5700 UVW) a few thousand miles. That being said, I'd recommend something lighter, and with a better suspension. The Nash has a traditional wood frame, leaf spring construction, and is t very aerodynamic. A more modern TT of similar size (25-26ft box) would be even easier to tow, and get much better mileage. My mileage was cut in half (13-14mpg), vs standard 28-30 highway.