> can't connect the 12V battery into the A2D of the PIC

Use a high-value (100s of k or megohms) resistive divider
into a high-Z input, low-power op-amp (eg TLC271, 10uA
operating) to give you the drive for the A2D. Op-amp Vcc
is from a PIC pin so it can be turned off to minimise drain

Could even use a FET / transistor to disconnect the divider too

However, as the regulator is a hungry hungry hippo 7805
(Iref ~ 5mA), a few 10s of uA isn't really going to make that
much difference, so you could go with a lower-R divider,
just enough to meet the A2D input Z

Different if you wanted as complete a power-down as possible,
then you'd be looking at regulator power management too

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