It's been done. Some folks kind of went the other route, doing primary wiring for 12v with some supplemental mains wiring for appliances that were hard/impossible to get in 12v. See old issues of Mother Earth News and similar alternative lifestyle publications.

There are some things to consider, the first of which is proper sizing of conductors - at 12v DC (more like 14.5 or so from lead-acid batteries), the amperage needed for a given amount of power (or light) is huge compared to 120v AC. Amperage, more than anything, drives conductor size. Look at the gauge of your starter motor cable. But while a 12 ga wire might safely carry a 20 amp AC circuit in your house, it may be undersized for a 20 amp 12v DC load because: In addition to the ampacity considerations, factor in voltage drop - long runs of 12v DC have to be up-sized to prevent voltage drop. And that 20 amp 12v DC load is only 240 watts.

12v will start a fire just as readily as any other voltage, so if you run wires for 12v power, I suggest that you put properly-sized over-current protection at the source - automotive fuses would be the simplest and cheapest way to do this.

Secondary batteries, particularly lead-acid, require maintenance and eventually need to be replaced. A whole topic on its own, but it can be a significant cost and PITA, which is why the current trend in PV and Wind home power is to use the grid as your "battery" instead of how Izzy's friend does it with a battery bank. Grid banking does us no good in an outage, of course.

In a bank that is wired in series to give higher voltages, one bad cell (or battery) cripples or takes out your whole system, depending on the exact failure. In systems wired in parallel for more ampacity, a bad cell or battery drains the good ones - one reason dual battery vehicles (like my trucks with Warn winches) use a battery isolator to keep the two (or three) batteries invisible to each other - I suspect that RVs with an on-board deep-cycle battery have a battery isolator wired in, regardless of whether or not one can readily see it.

There's the whole topic of venting gas, acid fumes, etc etc - best to not just stick some LA batteries in a closet or garage somewhere. These and other considerations are why almost no battery-backed emergency lighting is powered from a central battery bank - the batteries (and individual chargers) are located right at the fixture.

I don't think it is a bad idea, mind you. Just cautioning that there are few things to research before jumping in and doing it.

Regards,

Tom