In my humble opinion, based on my experience, towing a 40' 5'er with an 8.1 liter GMC, the best way to improve your mpg is to slow down. I was getting around 5.5 mpg while towing at 70-72 mph, I slowed to 62 and my mpg went to 9 - 9.5. I really wasn't expecting anything that dramatic, and it really didn't make that much difference in my daily miles traveled. A side benefit is that it made the trip way more enjoyable. It's funny how much easier the traffic is when they are passing you instead of you passing them.