This achievement is pretty simple.
There will be alot of arguing among the group while you are in the RV, mainly with Lilly dishing out accusations. You have to stop when the RV hits a walker, and the argument continues outside.
Deal with the events that unfold and once back on the road, go and speak to Katjaa
Then go and speak to Clementine.
When you reach the train blocking the road, you will get the achievement.