I agree with soundman - there are some great dc-dc converters with low standby currents and the handshaking required for graceful shutdown - ie, UPS functionality - PSU (dc-dc conv) sends "battery low - shutdown imminent"; PC starts shutdown; PSU shuts down when load drops or after a pre-adjusted time delay (or when battery is too low - ie, it collapsed quicker than anticipated).
I have devised similar timed handshaking using the MW728 (it triggers a timer that hold power on), though just now I wondered if another great HVTO/LVCO kit can be internally tapped for handshakes. (The kit is the $22 Oatley Electronics (Australia) K227 - 12-24V Dual Battery Adapter For A Vehicle, but you need to solder & assemble and house it. But it has an adjustable voltage level & better delays than the MW728; uses a latching 80A relay; and only consumes 50uA else 500uA (0.5mA) with a LED. An has other potential mods given its circuit & operation description - not that I fully understood it!)