You are right that Google Docs is not a good choice here.
You need to store this information on a private server, or maybe use one of the online services like pastebin.com
Retrieving it from C3 should not be difficult, you just make an AJAX request. The data itself may be in clear text format, for example "Today we are at XXX street", or JSON with map coordinates etc.
The most important decision is how the truck driver will be updating this info every morning. In case of PasteBin service they will need to login and edit the text. If you store it on a private server, you might create a simpler way to update - for example a small webpage or a Google Maps widget.