OK, next is to reduce the number of variables in order to determine which one is the problem.
Have you checked the system voltage? I assume that the engine was off when you had your problem. If so, what is the voltage at the battery and at the power source you tapped? If the engine was running, compare engine off voltage to engine running voltage. If more than 14.5V with the engine running, investigate that first.
If system voltage is OK, then see if hooking up the power outlet with nothing plugged in causes a problem.
If the power outlet is OK, then try one charger at a time. Try with no load on the charger, then with the device plugged in. If they work properly one at a time, add another. Repeat.