I understand what the OP is going after, maybe I can contribute.
We tow with a 2011 Honda Pilot, mostly in Florida, where it is flat. We noticed when pulling our 1971 Shasta that it was aero limited. Same issue with the Bambi.
Running behind semi trucks we'd get 15-16 MPG, out on our own 9-11 MPG. So I purchased a roof-mounted air deflector.
Without the deflector, at 65 MPH, we'd run with the torque converter not locked, around 2700 RPM, 10-11 MPG (US Gallons). With the deflector, 13-14 MPG, fifth gear, torque converter locked, 2100 RPM. I did some calculations and with the purchase price for the deflector of about $350, gas at $3.50/gallon, it would pay for itself in 4,000 miles towing, which we have already surpassed.
I bought my deflector from Icon Direct, a Canadian company. Fast shipping, good service, I'd recommend it. I believe since we bought our deflector the number of models offered is lower, but still check it out.