while i've not used the vehicle battery during an outage, we did use a car battery to supply all our electric needs for ten years.

it recharged our radio and flashlight batteries, periodically ran the 12V mini-tv/video-tape-player, powered the laptop (today that would be a tablet and cell phone), and even supplied juice for a couple interior lights (made from car brake-light bulbs).

i have read of a few folks adding a second deep-cycle battery (and some basic isolation circuit) to their vehicle that allowed it to be charged when they drove yet didn't draw down their main battery when the other electric uses were required.

eventually i built a separate charger made from a car alternator and a 5hp lawnmower engine so that we weren't dependent on the vehicle battery, which required taking a trip to build up the charge again. (i remember always parking the truck pointing downhill so that if we got too low, we could roll-jump-start it)

you might wish to look into what's available in 12V from RV suppliers. and these days small 12VDC-to-120VAC inverters have become pretty efficient & inexpensive, but beware, its all about the amount of consumption a product needs (ie: things like those 12V mini electric coolers will suck a battery dry in no time!)