Does anyone have a photo of a fifth-wheel hybrid setup? I just googled for it and found a modded insight:
Is it too late? - MIMA Honda Insight Modified Integrated Motor Assist Tour De Sol
The idea the OP proposed interests me personally more than the fifth wheel gag, but I honestly have no clue which would give more MPG. Would the electric motor use exclusively its own plug-in batteries or would it be connected to the "real" battery also?