I must have missed it but how long was the roadtrip for Rick, Carl, and Michonne? If it was a long trip, that's a pretty big trip to take while the prison could be under attack at any time and without the certainty of such things as gasoline and the availability of guns. If it was a short trip, how the **** did Officer Rick not know that the prison was right there when they were wandering around? That would be like living in Riverton and wishing there was a secure place like a prison somewhere near and wandering aimlessly hoping to stumble across it.
It does not say but the episode takes place in the course of a day and it does not appear that they took any camping supplies with them.