Battery drain

My battery drains completely out in about forty eight or less hours. The battery checks good, I have even switched batteries twice (have warranty battery). The alternator was change beginning of August 2017 and still checked okay by a professional. I change old radio and wires radio system for a brand new one by April 2017 (checked okay as well after that). I changed transmission on April 2017 (still running well after that). The connections on the battery all are clean and tight. Fuses are all tight. I even add a separated (on-off switch for the radio can turn off while SUV is parked or not in use). Checked vanity lights, dome lights, hood light, all off.
My SUV still in parking lot for a week and half. Another strange thing is that happen before: after truck is warm up and shifter moves to D (drive ) or R ( reverse) or any shift the battery volts go down less of half. I am frustrated, and overwhelm with this. I love my truck. Please help.