20-30 kWh battery pack
5 kg Propane range extender (holds 460k BTU or 127 kWh of energy - think BBQ tank)
15 kW Propane fuel-cell - 50-60% efficient - so adds 60+ kWh of range
One might be able to get the weight of the range extender down to 100 lbs - so the weight penalty would be minimal. Make the tank removable so you don't have to lug it around when you don't need it and it's easier to fill up. You can also use the propane to provide heat in the winter.
Now about getting something like that small enough to fit into your car... I still like the idea of a small trailer - built properly it could reduce aero drag for those long trips and also provide room for additional storage capacity.