This happens sometimes, a methodical approach is what you need.
Leave the bonnet open with everything switched off for a while then get your multimeter out and start checking at each fuse to see if you can locate which one has a current draw on it.
I take it the battery is a good make and plenty powerful enough.
Does it do mainly short runs or not?
Also the terminals are notorious for giving a bad connection. they need to be bashed on with a hammer and socket or something before tightening the stupid bolt thing. Ministry of Cr@p Design, i'm afraid.
and when you say the battery is "dead" do you mean it just won't turn the engine over on the starter or do you actually put a meter on it and find it is less than say 12.5 volts?
The starter solenoid contacts also go after a while.
Try charging it up, using the vehicle then when you stop, disconnect the batt.
(Lock the door with the key)
Then reconnect and see if the batt is still OK. If it is, the problem isn't the batt the terminals or the charging circuit. (Or the starter if it still works). You deffo have a parasitic drain of some sort.
If after charging the batt seems to be ok, you may have to keep on doing this rigmarole before you get to the problem. In lockdown I found this happening and it was simply down to lack of use. The alarm system etc was slowly running it down.
Best of luck