So it's been suggested that my inquiry is a bit misleading as I didn't specify that the gas mileage quoted was based on Canadian gallons, not US, and that there isn't a V10 on the planet that will do that kind of mileage. Over the past three summers we've travelled over 30,000 miles, so I'm pretty confident that the figures I've quoted pretty much cover what we get, from no tow, little tow, to flatdeck carhauler tow. I've read one account (elsewhere) of a Fleetwood averaging 6mpg with a little tow, doing 55mph, but I think it's considerably longer than what we're looking at. I certainly don't want to drop to 6!!
Judy