Thanks sailingdog, your prior post was one of the resources I've been referring to.
I am pretty sure everything is off. There are literally zero cables connected to the positive battery terminals, other than the battery interconnects from positive terminal to positive terminal. This is true for both the house bank and the starting battery. Is that what you mean by shutting everything off?
What do you think of the declining current readings from 90 ma down to 2 ma? Is that some effect of the multimeter itself?
I think my next step is to start flipping individual breakers and seeing if current is drawn by the device on the circuit even though it has no neutral connection at the negative/grounding bus bar (hence it is completing its circuit through the ground leak). This is what Casey recommends in his book.