Workhorse 8.1 liter poor fuel mileage

I have a 2005 Itasca Suncruiser 38R on a W24 chassis. I bought it new, a factory order. I weigh 23,200#s loaded. 32,100 miles. I towed a Honda Civic about half of these miles.

I've averaged 6.97 mpg since new. I mostly keep my interstate driving speed at 60-63 mph. I use cruise control.

-Tom