No power in Mountains

I recently bought a 2004 Prius with 104,000 miles on it. When I try to go up into the mountains I only get a few miles or up to 1,000 to 2,000 ft. elevation (from 300 ft. starting) and the battery level gradually drops from 6 bars to 1 bar. The red triangle light and check engine light comes on and the car speed gradually begins to drop even if I continue to press on the gas pedal. I have only gotten down to about 35 MPH from 60-65 mph before stopping and turning around. Within a few miles of turning around the light goes off and the battery goes back up to 6 bars and works good going down hill. The dealer I bought the car from has been working with me to figure out the problem but so far we have not figured it out. He has replaced the main battery as well as the inverter/converter. The mechanic was with me on a test drive to the mountains and had his car computer plugged in while we drove and experienced the same thing. It gave the code for the catalytic converter so we changed the entire CC/muffler (one long piece) and still got the same code. The battery indicator on this car has never shown more than 6 bars. I'm not a Prius mechanic but I personally think it has a bad "Gas Engine" or something wrong with the gas engine. How do I tell if the gas engine is bad or has a problem?