If you have multiple monitors connected to the device running RealVNC Viewer you can configure RealVNC Viewer to use all of your local monitors to display the monitor(s) from the remote device, when RealVNC Viewer is in fullscreen mode.
To do so, first connect to RealVNC Server running on the remote device:
Next, click the Properties button on the RealVNC Viewer toolbar:
Select the Expert tab:
UseAllMonitors into the Filter box, and set the Value dropdown to True:
Click OK. Click the Enter full screen button on the RealVNC Viewer toolbar:
RealVNC Viewer will now use all of your monitors to display the remote device's desktop!
To exit fullscreen mode, click the Exit full screen toolbar button again.
Using RealVNC Viewer for macOS?
You need to disable "Displays have separate Spaces" in System Preferences, Mission Control.
You also need to set UseLegacyFullScreenMode to True in RealVNC Viewer's Preferences, via VNC Viewer -> Preferences in the menu bar at the top of the screen and NOT via the toolbar that appears while connected to a RealVNC Server.