Mileage?

So you’re experiencing the bladder problem and it’s hard to know how much is in the tank.
Long term averaging of miles and gallons pumped should be reasonably close though .

get a free load test at auto zone

 

here is one post about the Gen 2 bladder.
Gas Tank Guessing Game | PriusChat

and another one here
Internal Bladder? | PriusChat

 

the bladder can shrink over time, and in cold weather. if you're only getting six gallons in from empty, it sounds like that.
the load test measures battery capacity in cca (cold cranking amps) and voltage under load.
best way to determine if it is good or bad.

 

if you're calculating the mpg after filling up, it doesn't.

 

It seems like your problem is related to This.

I recently purchased a 05 Prius at 140,000 and I kept all my refuel reading. I drive highway mostly ( avg 65-70mph, 1-2hr/day, refuel every 430mi). I live in southern us it's 0C winter and 20C currently.

Here's what I have experienced: there's no mileage/gas bladder size difference between seasons.
But on my first 2 trips after driving the car off lot to be 27mpg/32mpg.
Then it slowly climb up increasing 2-3mpg every time I refuel until it peaks at around 49/51mpg.

I'm averaging 50mpg right now.

Since you have mentioned that you drive it light, I would like to think it's because the car's computer need to relearn from driving harder on highway speed.