I have a cruiser and get <1mpg above 12mph. I have a camper and get <10mpg above 63mph. I only know these numbers so I can forecast fuel stops. Considering mpg and fretting over that number distracts from the enjoyment of using the toys.
Ownership and use lets me have slightly more control of my recreational options and lifestyle. No one said this was an inexpensive or cost saving hobby. Without intentionally lapsing into cliches but I have, it is a lifestyle or possibly better, a recreational style choice. Take more of your stuff with you and unpack less each day and repack less each day. Minor cost, mileage.