Oops, forgot a few things and couldn't edit post above.
No need to tap into the 12v side of the inverter outlet, just plug a usb cube into the outlet.
Or........On page 70 of your owners manual.
"The 12 Volt power outlets and 5 Volt (2.5 Amp) USB port (Charge Only) have a cap attached to the outlet indicating “12 V DC”, together with either a key symbol, battery symbol, or USB symbol.
A key symbol indicates that the key must be in the ON/RUN or ACC position for the outlet to provide power. The battery symbol indicates that the outlet is connected to the battery, and can provide power at all times."
OR......... check to see if you have a fuse in location F91, it is for ignition fed outlets. If not, you should be able to pull the F 90 fuse and insert it into the F91 slot, which should make your outlets ignition key powered.
I had switched and unstitched usb and cigar lighter outlets on my last 2 Caravans. The usb in the glove box was hot all the time, while the others were switched.
It would seem strange not to have this in your truck.