12v battery should I replace now

I have a 2010 prius and since August 2020 my mileage has dropped about 5mpg consistently and down to 39pmg at end of each fill. In August I got 4 new tires and oild change at diff places both chains Jiffy and NTB. I always have trust issues with the chains at they are notorious for not doing or doing more than you bargain for. In either case thats when the issue arose. I set the psi on tires at 35 and checked that afterward. LTS I read several items that indicate a bad 12v battery could be the culprit. I puch Prius used @ 130K and it currently has 162K. The 12v was replaced according to docs in the glove in 2016 so its about 4-5yrs. The DX in the dispaly indicartes 12v is anywhere from 10.9 to 11.9 when the car is not running and I know thats is low but havent had and trouble starting. WHen the car is running the coltage is about 13.6. I called the dealer(Toyota) but they always start with we need to DX and if we find that is not the issue you will get a DX fee which is always 100 or more. The 12v is still under a prorated warranty so there is a chance to get it replaced for about 170 from them if they dont play games. What voltage should the 12v read to know its a solid battery when the hybrid battery isnt running and is the 12v battery enought to cause the MPG drop. I will also add that mostly my hybrid batter is fully charged according to display except if I am running AC in hot weather or doing alot of park n go driving with car on. I notices the bars drop sig and the gas engine needs to constantly run but it recovers quick enough. The cae under eco and no mode also drives more sluggish when not going very fast etc