what causes power loss during towing?

I agree. It's at the repair shop now for diagnosis. I had understood that the engine retards timing when lower octane fuel is sensed, but I have no conformation of this. Tire pressures were monitored continuously and tire and hub temperatures checked at every stop. The best observed mileage was thru Tennessee (I-65) at 14.9 mpg with the first fill of 93, up from 12.4 in southern Georgia. The worst was 11.3 with no-ethanol 93 in WI on the last leg of the trip.