i gotta do something besides worry about it. i followed a guide i found on here to get those battery readings from the center console. figured that was the proper thing to do but mendal said those measurements are meaningless use a volt meter. you are now the second person to recommend the volt meter. it's either buy the volt meter buy a battery or do nothing. it's not like i have any other suggestions at the moment besides charge the battery. to be honest i am considering that but wasn't sure if it was detrimental to charge a battery that tested good.
now that you've clarified the 10 mpg drop, you'll probably get different advice. are you aware of the gas tank bladder issue?
what is a proper charger? i was thinking about charging it at home with a trickle charger. i am not aware of a bladder issue. pretty much everything i have read suggests tires and 12v.
trickle should be fine. gen 2 prius has a rubber bladder that expands and contracts on a whim. single tank results can be misleading, and only long term tank averages are close to accurate.