I agree with the battery isolator as number one suspect, but isn't there an "explosive" disconnect device on the positive cable terminal that severs the high amperage connection to the battery in the event of a severe collision or rollover? Might it be possible some corrosion has developed internally in that?

2002 X5 3.0 303,700 miles
2014 428i 19,000 miles

2004 325i sold at 123,600 miles
2001 325i sold at 66,000 miles

1970 Firebird - Undergoing restoration