Recently towed a 11,000lb 5th wheel from Ontario to Arizona and back, a total of just shy of 8000 miles (with side trips). Had the oil changed before we left Ontario and had it changed again in Benson AZ after covering about 3,500 miles. At this point, the DIC was showing 60%.
Our return trip was longer and we drove much harder, in fact towing at between 70-75 mph on the interstate. By the time we got back home, the DIC was showing 9% (the dealer in Benson forgot to reset it). I reckon that I could easily achieve 10,000 miles between oil changes if I went by the DIC.
Incidently, we averaged 11.9 mp(US)g when towing and 21 mp(US)g solo.