new battery is the first thing i would attempt. it might have a faulty cell which is draining to earth (short)
if possible get hold of an amp clamp, with the engine off see what the drain is, average multimeter is 10amp limited so if there is a substantial drain you'll blow the fuse inside.
to give you perspective a single 5w interior light bulb draws 0.5amps.
much more than that and the calculation to work out ur battery capacity becomes relevant.
your battery will have a reserve capacity on it.
thats the load it can take before it becomes unusable.
if rc = 50 then it can take 50 amps for 1 hr or 1 amp for 50 hours etc. (all quoted on a BRAND NEW condition battery)

my money is on a shagged battery