I've kept a inverter i the truck for years, it is handy. Efficient, no. Efficiency is better if you use bigger batteries, several golf cart batteries, and use the engine to charge the batteries and turn the engine off and run down the batteries. And golf cart batteries are designed to be cycled this way. If you ever add extra batteries to your vehicle make sure your alternator is rated for heavy duty use. In most cars this is a good way to smoke an alternator, and no i didn't do it but i have a friend who did.

For the best efficiency driving a generator from your vehicle engine is the way to go. The systems, direct drive and hydraulic, are fuel efficient and durable but expensive.