Geeesh, the dealer replacing the relays and having it work for a bit does sorta imply that the relays' are failing and in a arcing/burning closed mode too.....
From the description of the circuit, the master switch runs/powers these relays coils that are used as cut-out relays' that when closed is power things via their N.O. (normally open) contacts due to that they're in series with and are switching the main power that feed/s to all the accessory switches........
Thinking if they do stick closed, it would drain the battery if ANY of the devices being powered by the accessories switch is left on or has a standby current........ Wondering if they are in fact arcing closed due to a poor design?
Does the battery still go dead even if you shut off all the accessory switches along with the master switch being off?
If not? If those relays are sticking on and there is some current draw in some of those devices, I'd be inclined to open up the relays and check its contacts for molten pitted metals in those contacts.....