What is causing my battery voltage decrease from 14.2 even with new battery?

That’s perfectly normal after engine has been turned OFF1 - the alternator is what increases voltage while engine is on.. Engine ON voltage should be approximately 14 volts at battery terminals when testing with a volt meter. It is unclear how you’re actually obtaining these readings - however testing should be performed at the battery itself. Good luck!

In agreement with Pushrod and to clarify more -- the battery voltage while the engine is off should be somewhere between 12.4 and 12.7 volts. While the engine is running at 2,000 RPM, you should be around 14 volts or so. Again, definitely lots of variables not knowing how your readings are being obtained. Keep in mind voltage is not amperage -- and does not weigh in on the ability for the battery or charging system to handle a load.