Don,
We bought our Duramax new. Our mileage varies somewhat due to our location & weather (wind!) and the only flat terrain (from here) is heading east into west Texas
Typically we get about 10-12 towing, and 18+ Hwy. We're in a rural/isolated area, so we rarely travel the Interstate without the 5th wheel in tow. Our Laredo usually weighs
about 9,200+ lbs packed for a 2-3 wk trip & less for shorter trips (of course). It's just the 2 of us & the 4-legged family members. Hopefully that will give you some comparison.
Please keep us posted on your toilet 'fix'... those of us with the same toilet would like to know if it works.
Terri, the Chevy co-pilot