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I assume you are wanting turn by turn from an app running on the iPhone. What app are you using? How are you connecting the iPhone to the truck?
I believe any display capabilities are limited to Apple CarPlay compatible apps and I believe the 2021 UConnect (version 4) is limited to only enabling Apple CarPlay and Anrdoid Auto when connected by USB cable. Version 5 which may be in the 2022's is supposed to enable CarPlay/AA via bluetooth.
The UConnect's built in navigation should provide turn by turn directions. I believe these can be configured to display on the instrument cluster, HUD (if equipped), or the UConnect display. If you're trying to use the built in navigation and it isn't showing turn by turn, I can probably dig up where the settings are.
Current RV Information: 2018 Forrest River Salem Hemisphere 282RK
Previous RVs: 2004 Fleetwood Bounder 32W; 1999 Four Winds Five Thousand 21RB; 1986 Allegro Bay 27'