To measure the drain:
Top tip – wear goggles if you’re scared and wash your hands after handling the battery.
Leave the car parked for 15mins with everything switched off and locked, but with the hood open.
Loosen the nut to the battery negative clamp so it can be slid off. (tip, don’t short the battery terminals, you’ll get burned…)
Set your multimeter to 10A and place the wires into the 10A socket and negative
Arrange your multimeter probes with the tips positioned such that you are able to slip the negative clamp off and then have the current then flow through the meter from the bare terminal to the disconnected clamp (you seamlessly insert it into the circuit)
Observe the drain current. Probably 20,30mA. If, say 100+ continually, something is probably wasting power
But I think you just don’t drive long enough to maintain the battery charge.
If you have a multimeter, use it to monitor the battery voltage over a few days. Look up lead acid voltage to charge charts…. (tip, do switch the multimeter wires out of the amps socket)
I think you’ll need shoes, or a trickle charger of some kind.