As mentioned C5 and never vehicles have many controllers that stored settings and draw amperage when vehicle is OFF
depending on the weather the draw can be even higher
The first battery on my 1999 Corvette only lasted about 4 years and then one cell got weak so battery never fully would charge up
Since then I began using a battery tender that only when in charge mode put outs about 1 amp so battery cannot get cooked.
The second battery is still been good all these years
You may want to check that the little relays used and there is several of them is not stuck in a on state by moving the functions
like seat, outside mirrors, HVAC, headlights, etc would cause forcing relays to change state.
If you do not have one, maybe a auto parts store would let you use their battery load tester tool to assure all cells are OK.
You do not want the battery to get too low as that would cause not just settings but also the adjustments the controllers for
engine, tranny, ETBCM, etc to erase all stored settings.
In my case I ran wiring from battery to outside the car and can simply pull tender in without having to open, door, hood, etc