I found it! In order to get the caravan guard chat option you have to see the caravan. Go to the Track (I assume during the day) and take a short break. You should see the caravan go by and have an interaction with the guard. After that you can talk to Harold.
And you need to sneak into the camp at night to get the scene with Tank and Scar.
After those 2 events it should progress. (and at some point the option to "sneak" in during the day comes along)