Don't plug the phone into the truck usb, the headunit is reading it as an ipod.
So the only way you will hear the prompts is to set your headunit to ipod mode.

You've got 3 choices, use a 3rd party charger so the headunit only sees bluetooth from your phone and never thinks an iPod is connnected.

Check the "Play as Bluetooth phone call" option on the Google Maps app. The problem is the delay between the phone and the headunit which can result in late directions.

Or uncheck the "play voice over bluetooth" option to force the prompts to be played from the speaker on the phone.