Check the alternator. What you are describing sounds EXACTLY what just happened to me. Simply get a multimeter and put the + on the + side of the battery and the - on the -. It should read (idling) between 14v and 15v. Then turn on the lights and see how much it drops.

__________________
2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k