U0121 possible fixes

2013 GMC Terrain 15k miles barely ever driven brand-new condition kept in
garage no rust no corrosion no dirt immaculate. BRAKE ABS TRAC lights on,
Service Power Steering message, lane depart and collision alerts not
working 3 brand new fully charged tested-good batteries kept dying overnight
in 36 hours. Took to dealer DTC U0121 pulled ABS module fuse to stop
battery drain but told me it would cost $2000 to replace EBCM and
associated parts but even then wasn't sure it would solve the problem. Need
to know if there are any other possible causes, solutions, recommendations
for this nightmare. Never expected such a catastrophic problem from a car in
this new condition which isn't under warranty due only to time.