When you say "to keep the battery charged" do you mean you want to use the DC-DC converter to power another 12v battery? Depending on what type of converter you get you may not even need the 12v system battery, if the converter puts out say 10A at 12v it may be enough power to run all of your lights. you wont need the engines starter anymore.
keep in mind that if you keep the 12v battery in the system and power it with a 12v DC-DC converter you will need a 12v charging circuit added into the mix, or a 12v HVC (high voltage cutoff) to stop the converter from putting too much current into the 12v battery when it does not require a charge.
GM 48v 1000w first gen.
X5404/Genesis Farfle/Lyen 12-fet/20s Lipo/CA v2.23