SOLUTION
In dom0 terminal you should opening your Whonix-17 TemplateVM terminal, or Whonix-17 AppVM Terminal as root using this syntax:
qvm-run -u root Whonix-17-TemplateVM-Name xfce4-terminal
or
qvm-run -u root Whonix-17-AppVM-Name xfce4-terminal
Try that and you should be logged in as root in Whonix 17, and be able to run commands (without having to use sudo).
For more security, you can log in as “sysmaint” user with this syntax:
qvm-run -u sysmaint Whonix-17-TemplateVM-Name xfce4-terminal
or
qvm-run -u sysmaint Whonix-17-AppVM-Name xfce4-terminal
This is a bit more secure, and will log you in as sysmaint user, allowing you to run commands (also), but you need to use sudo for commands that require root privileges.