On the west coast, vehicles towing usually have a 55 mph speed limit on the freeway, and I stick reasonably close to that, sometimes to 60 on the flat but rarely faster. I've slowed down not towing, too, and stick closer to the speed limit, though I have not seen a huge increase in mileage. But I also notice a lot of other people have slowed down, too.
I think vacation time is a little different, though. If you are going to work, you can leave five minutes earlier and drive more slowly, but if you have only 9 days for your vacation you want to get there fast and are usually willing to sacrifice gas to do it. When I drive trailer-less to Washington in one day, I want to get there and it is worth a few dollars more in gas to get there faster. When I'm going somewhere 40 or 50 miles away, I don't mind it taking 10 minutes more.