Although those PicoUPS look fairly economical, an alternative may be a 2nd battery which is only connected (to the primary battery and charger) whilst the vehicle is charging.
See f.ex mp3car's Smart Battery Isolator (Post#7) - and for derived circuits - Using a relay to auto-power on HDD and power inverter (page 2).
For a low voltage cutout to protect the 2nd battery, you could use an MW728 "Battery Guard" (~$20).
Although the MW728 switches 10A, by having it switch a 12V relay instead, series diodes can be inserted to increase its 11.2V cutoff voltage upwards in .6-.7V or .2-.3V increments etc.

