I've had the same battery since I bought my 06 , I thinik its the oem.
5 yrs later, it still seems fine, but my MPG has dropped from 34 to 28.
Could a weak 12 battery cause this, ??
I realize its only used for small amp draw accessories, but could the larger HV battery be getting drained more to compensate?

No check engine lights or indicators of any kind...

 

First test your 12v battery and then make a determination on whether it needs to be replaced (I would). Unlikely the 12V battery would cause a drop of that much in mileage.