Thats really curious about the very low charge rate (amps) through the aux battery charge ckt. Its been a while since I pulled a TT, but the old Aux charge soleniod for that circuit would charge at whatever rate the Alt would put out for the battery voltage. The Alt was capable of putting out a charge rate that would charge a really low Aux battery. The solenoid was a continuous Duty metal case and would provide isolation when the vehicle was shut off.
Do the new vehicles have some current limiting device in the Aux circuit? (I dont have a new EVTM---hopefully soon will)