I have come to the conclusion that my 12 volt battery's voltage at 12.2 since new is to low. Toyota dealers say it is good. My vehicle gas malage has gone down to sometimes 28 mpg to 38 mpg. When driving about 20 minutes the Internal Combustion Engine (ICE) fail to turn off especialy with hot weather of 105F. Dealer says its normal. Has any one had the same problem?
Battery sticker says it has 345 Cold Cranking Amps. Parts at Toyota say my car needs 540 CCA.

 

I have no direct experience with the Rav4, but I have experience with hundreds of Toyota hybrids. The 2021 Rav4 uses the same 6S NiMH module as many other models including the Prius.

12.2V is good or bad depending on conditions. Sitting there on a bench 24 hours after being fully charged with no loads whatsoever, yeah, definitely bad. If in the car with light loads on it (interior lights, computers, etc.), it's good. If in the car charging, either the battery or the inverter is bad.

CCA is irrelevant to your vehicle. It never cranks anything. It simply provides the 12V power to the car for the 1-2 seconds it takes to connect the HV battery and engage the DC-DC converter. The hybrid battery starts the car. It's common for the 12V in Toyota hybrids to last 8+ years. I recently encountered a 2013 Prius C on its original battery.

Mileage is very subjective and far more likely to be an external factor. The hybrid system becomes progressively less utilized as battery temperature increases. Living in Phoenix, I'm very accustomed to the symptoms you describe in a properly functioning hybrid when temperatures become elevated... We had 70 days over 110°F and 113 CONSECUTIVE days over 100°F.

It is critical the battery be kept as cool as possible. The A/C needs to be run to keep the ENTIRE vehicle cool as though every seat were occupied. If every occupant (even if they're just imaginary) isn't cool and comfortable, the battery is getting too hot and being damaged at an accelerated rate.