Towing Speed / RPM dilemma

Higher engine speed does not always mean more fuel used. Many people that tow RVs in your situation actually use less fuel using the next lower gear because engine load is lower. Engine is operating at a more efficient speed/load point. You don't have the vehicle slow down, downshift to speedup cycle either. Many people that tow frequently or drive in the city change gears (to numerically higher axles ratio) also use less fuel. Regardless, any difference is fuel used will not be measurably different. Your trans temp and driving experience will be.