Do the house batteries discharge too?
If you have an older diode based isolator, it may be worth checking for forward and reverse bias (or just change it out to the Blue Sea System ML-ACR which is a better setup). Optimas have a higher standing voltage than many other batteries, so I'd be concerned about leakage through with a diode based isolated if the house system is lead acid anyway.
I'd start by separating the starting batteries from the house system anyway (divide and conquer). You will likely want to perform a parasitic draw test, which amounts to putting a decent amp meter in series with your ground wire and removing various fuses to see where the offender is. There are lots of Youtube videos on this. Keep in mind there is likely more than one ground with a dual battery setup. My 7.3 has one on the starter in addition to the chassis. One can be disconnected entirely for testing.
There is a great post by Scalf77 on (house) wiring setups here -- >
https://www.sportsmobileforum.com/fo...ches-7991.html
Good luck finding the gremlin!