New 12V battery not recharging in 2015 Prius

Problems with the 12v auxiliary battery are quite common.
Short trips do not have time to charge the battery, which after the car is turned off continues to actively consume energy by various systems.
What battery did you use? Manufacturer?

 

All you want to do now is confirm you are in fact charging so you have the 12 volt open in the trunk of the car and the hatch up and you measure the voltage across the battery terminals the two round silver things on top of the battery You should be staring at 12 volts or 12.5 volts for a fully charged AGM battery any lower and you're falling into a black hole fast with low voltage. Then go up and ready the car all the lights on on the dash the car says it's ready now go back and put your two test leads on the same two terminals you just had them on looking at the voltage we just talked about It should be 13.5 to 14.5 with the car readied If you don't have that you probably have a charging problem to the 12 volt because right now in this mode you should be charging your 12 volt there is no alternator to turn around and around no belt on the front of the motor remember so this is being done by a circuit in the inverter that's taking some of your 211 volts and stepping it down to 13 5 to 14 5 volts to charge the 12 volt battery If you don't see that you don't have that and you're not charging. If you do see that and you have that you're not driving the car enough All this has been documented for well 10 years with this model so you're going to need one of those little silly chargers you put on your motorsports equipment like scooters and snowmobiles and whatever else you might have where you live motorcycles and things a little one to four amp trickle charger that you'll attach a harness to your battery drill a little hole in the plastic plate push the little plug up through the hole so you don't have to go through all this every time to access this plug you just installed and you plug that to your battery tender which is the thing you bought to keep your battery topped up and you plug that bad boy in during those 6 weeks or when you're only driving 5 minutes every month that might help you You can see how this is done by googling attach a battery tender to my motorcycle it'll show you a smaller battery how they put the harness on how they put the harness somewhere where they can access it with the plug that goes into the outlet in the wall and all of that and you'll do that to your car. And maybe never have a problem again until the next thing

 

You could have a defective 12v battery. Which brand did you get? OEM Toyota battery?
Or something else?
When the car is "ready", what is the voltage to the 12v battery? It should read between
13.5-14.5 volts. If it's not, there is an issue with the inverter.
How did you have it checked? Who checked it?

If you don't drive the car much for 6 months, the battery will not get a complete charge.
You should probably charge the battery for once a month or so.

When you drive cross country, when you get to where you're going, do you go back to
short trips? Bring your charger....
If you drive for 3 or so hours straight, that should be enough to top off the battery.

 

Stupid question, but did you put in a vented AGM battery like was originally in the car or a flooded or MF battery? The car's charging system, expects an AGM battery and the hydrogen gases must be vented outside for safety reasons.

 

You aren’t driving enough, keep a maintainer on it

 

.
IMHO; things need to be properly tested - rather than guesses with blind replacements. I realize that testing cost money but that's the ONLY way to diffidently nail down a problem. I know it's cheap to replace parts and if the problem goes away - the assumption is that was the problem. Until you run into something like what your experiencing.
I'm guessing you have a parasitic draw in your system. If your DC-DC converter wasn't working, you'd be dead on the road somewhere within a few hours - faster if the headlights are on.
Take the car to an automotive electrical shop. They should be able to nail down your issue. If that's too expensive, you can just keep it on a smart 2+A trickle charger. Keep a fully charged jump pack under your seat for those cross country assignment.

Good Luck...