MPG drastic fluctuations driving me crazy

I just bought my second prius with 153k miles on it, it has 157k on it now, it's a 2010, pretty sure it's the base model. I know it's not a plug in. My MPG is going whack.. It goes back and forth from 50 mpg in the city (at all times aside from this issue arising) to 30-40 mpg average and. My driving habits are very disciplined, I am constantly using the gliding technique..

I had my hybrid battery stress tested (condition was good) at a specialized shop, and I had my 12 volt battery tested, they said it needs to be replaced, which I don't think they were trying to up-sell or anything because they don't even replace the 12v batteries at their shop.. $150-200 fifor the 12v which I will be doing as soon as I have the funds available.

I also noticed that it seems whenever I'm getting the 30-40 average mpg, when I try to use the glide technique, it seems much harder to do if not impossible, so I don't know if the soon to be bad 12v might have something to do with the hybrid battery communicating with the engine/computer causing the main issue or what.. I'm really hoping it's just that the 12v in the rear needs to be replaced, but I'm completely lost.. maybe all the computer readings are wrong and my mpg isn't changing.. I'm clueless. Any help would be greatly appreciated. Thank you <3

 

Thank you! I'll be sure to check out the recalls and test my mpg at the pump. Thank you for all the advice!

EDIT: I just checked my VIN number and there does not appear to be any recalls as of now.

I know it's the original hybrid battery..so 9 years old I think. I don't know if this is the original 12v or not so I'm really unsure about the age for that one.