Ben, If batteries are in deed good and fully charged, sounds like you may have parasitic drain/draw on batteries. To help diagnose this with a DVOM, with vehicle off, hook DVOM to battery, pull one fuse at a time till you see voltage reading a bit higher and then you will know which circuit is drawing power and can correct or some how put a switch on it when not needed. DVOM can also be used in amp setting. For more methods and complete instruction on troubleshooting parasitic draw view the links below. Good luck.

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https://www.youtube.com/watch?v=KF1gijj0...

https://www.youtube.com/watch?v=yFCT-YZb...

https://www.youtube.com/watch?v=QRso1A0V...