My sister got an electric car and left us her 2012 Camry. We’ll probably sell it but for now it’s just going to be a backup car that she’ll drive when she comes home (she lives far away) for a total of maybe 2-3 weeks a year. I start it up once a month to keep it charged. Since it’s not going to be barely driven, what kind of fuel stabilizer would you recommend? Or is it even necessary? And should I fill up the gas tank?

Normal gasoline has 100% efficiency for about 6 months. It can be used up to 2 years. But problem with it is over time if not used it creates gunk. So more advised would be to fill it up 1/3 gas tank and use it for a drive, and every 6 months fill it up. Or you can just fill it up full and use fuel stabilizer. I dont know for how long fuel stabilizer will help.