Volt and Temp Gauges Fluctuating and Reading High

[dmarty]

My 2012 5.3 Sierra with 195k miles started showing temp and volts greatly fluctuating high while driving at highway speeds the other day when outside temp was -15F. No codes or IP warning light. Tried replacing battery charge sensor, hall-effect unit on neg cable with new GM part, alternator is one year old GM unit but tried switching with known good unit, replaced coolant temp sensor with new GM unit, thermostat is one year old but replaced it as well. Cleaned cable grounds in engine bay. Problem still present. Coolant gauge is wrong, as point and shoot laser gun shows normal engine temp (180-195F). Battery volts measure with meter at 14.7-15.8v at idle so battery volt gauge in IP is correct. If I disconnect sensor wire at alternator battery volts drop to 13.7v and IP warning says to check charge system. High coolant reading causes both rad fans to run so pulled fuse but batt volts still measure high. Coolant is GM orange and one year old. Hydrometer says good to -35F. Now that outside temps have increased to +40F coolant reading is closer to normal but volts are still high at 14.7v. Battery is 2.5 years old so haven't tried replacing it, don't think it's a likely cause, but may try it. I've tried everything I can think of and am starting to think the body control module is failing and causing alternator to overcharge battery which is causing an incorrect temp reading. Any suggestions before I take it to a pro with proper diagnostic equipment? Thanks!