Replaced my HV Battery from a low mileage 2013 on my 2006 Prius.

no problem, just report your post and ask mods to move it.

hard to say if hv battery was an mpg problem. at 220k, it could be a lot of things. how is the 12v health? what are you running for tyres and pressure? is it a hundred degrees everyday?
do you have records of mpg's as measured at the pump by date? what were they at their peak, when did they start to decline, and how long time/miles until they bottomed out? does she burn much oil?
i would wait a couple tanks before making a judgement.

 

I haven't read how long it takes the ECU to adjust to your driving style, but it's fair to expect lower than ideal fuel economy while it's in transition and I'd guess it takes a couple thousand miles to get there. Your tires could also have an effect on fuel economy if they're not designed for low rolling resistance or underinflated. What was your MPG before you started to have battery problems?

 

There's a lot of ICE items that should be addressed by 220K miles: spark plugs, mass airflow sensor (MAF), throttle body, PCV, engine air filter. These should be done every 100K miles, I'd say.