I initially was thinking of using the 5th wheel to do gas off acceleration, but even the e-tek motor, with a 4:1 reduction,and 200A draw, the acceleration was slower than I was comfortable with. In normal traffic I tend to accelerate with the gas, then turn off the engine and cruise with the 5th wheel, if the traffic speed is less than mu 30 mph EV max. My record is 42 miles on a full charge with an average speed of 26 MPH, on a deserted secondary highway.
Also check out Hybrid adapter:
http://peswiki.com/index.php/Hybrid_Adapter
This concept could turn any car into a hybrid.