We connected a new radio/CD player to the RV battery and the engine battery did not start after doing all this. We boosted it & it started right away. Today, we did the same to connect an inverter to the 12V battery & the engine did not start. Had to boost again. Both batteries are brand new. If we just play the radio & put the lights on, the engine starts o.k. It's only when we are doing work with the RV battery (12V). Thanks.