Low mpg in 2007 prius - talk me through diagnosis

Hi everyone! This is my first post here. I have a 2007 prius, which is my first and only car. I bought it at 130k miles and it's now at almost 190k. It came from Texas after hurricane Harvey and had water inside it, but not in the engine. Original hybrid battery but I haven't tested it. When I bought it, I averaged 45mpg. Now I struggle to keep it at 40mpg, usually it's closer to 35. It also burns oil, I usually have to add some in between changes.

A mechanic has given the car a once over and nothing major is wrong, but I'm sure there's a hundred fans and belts and things they wouldn't have checked without good reason.

Where would you start? I'm thinking I'll check the tire pressure first, then the 12v battery, then the hybrid battery. What else would you check - and if you have a suggestion, please tell me how to check if I can do it myself!