Prius Prime fuel gauge problem

So, is your gas gauge showing a full bars when the tank is full, and empty when it is about 2 gal left in the tank? The number you are citing is not a gas gauge but DTE (Distance To Empty) range, and it is dynamically "estimated" by recent gasoline consumption efficiency (mpg on HV mode only not including the EV mode in the calculation). If you drive your car very efficiently on HV mode, then you will see longer DTE on your next fill-up.

See this thread:
What is your DTE after fill-up?

 

What was your reasoning to fill up the fuel tank when the check engine light turned on?

Getting 418 miles DTE seems pretty low. Do you drive up lots of hills or travel at fairly high speeds? My 45 mile round trip commute has me driving up and over a few hundred foot hill each way and doing ~70 mph on the freeway, and the DTE after a fill-up is around 500 miles.

 

the problem could be related to the check engine light, but since disconnecting the the 12 volt battery that code whatever it was might be harder to impossible to find with a cheap code reader.

I doubt anyone would be able to give even a best guess what the cars problem is from the description given in the OP.

If it were me, I'd be much more concerned with why the check engine light came on, than why the cars distance till empty estimation dropped from whatever it was before disconnecting the 12 volt to it's 418 miles till empty estimation showing after the 12 volt battery was reconnected.

 

The DTE change makes sense because you've basically reset it's computer and you did so in winter. All of the available data it's going to estimate with is going to be lower gas mileage winter driving.

I do wonder if the OP meant to say that the gas light came on way early which explains the stopping for gas and the dealership just going with a reset.

Is the gas gauge and gas light performing as expected now?

 

The OP may have read a article about loose gas caps triggering a CEL; but I agree with everyone that just disconnecting the battery to reset everything isn't the correct solution. The error codes should've been read; so you have an idea of what potential problem you may be facing.
Lots of rain in S. Cal the past few weeks; which requires higher gas/EV consumption to move the car. This equates to lower DTE. Now that the car has been reset; expect some weirdness while the engine systems recalibrates itself.