Something is drawing the batteries down, I doubt it is a bad ground, and I am sure the EX is different from the Superduty so the easiest way is to just follow the ground cable, shouldn't go far!!
But to test the system, start with fully charged batteries. Take a volt reading at the battery, and then start removing fuses one at a time, when you see the volts jump up, you know the drain is on that circuit, then it's time to start looking at what's on the circuit. I would start with any aftermarket stuff that has been added, like stereo, alarm, etc... My buddy bought a truck and the PO wired the stereo to be on all the time, but never told my buddy who hardly listens to the radio, it just had it turned way down, but itwas still drawing and leaving him with dead batteries. Good Luck
NCH