Plug in car w/propane (LP) generator

Plug in electric can handle most all of my distance needs between charges. However for emergencies or extended trips, I would like to see a small propane (LP) generator to extend mileage or recharge battery if no outlet is available.

Why LP instead of gasoline?
Less mtc than a gas engine, yet can use the same basic design.
LP burns cleaner so emissions control for a small engine are less complex and less costly.
If a gas or diesel engine is in a plug in hybrid and that internal combustion engine isn't used on a regular basis, the fuel can get contaminated.
LP is easily and readily available. Vehicles could be designed with a safe location accessible in the trunk that can hold a standard 20lb tank like those used for gas grills. These tanks are quickly available for exchange anywhere in the US at prices comparable to gasoline.
Propane delivers very close to the same amount of energy as gas so a 20lb tank, about 4gallons, should be able to add another 150 miles to the range of a plug in hybrid when needed. If you need more just stop into at a convenience or hardware store for an exchange.