Drive back to the overrun Refugee Camp where Sarah’s Memorial Stone is located. Every time you visit this location back, always expect a handful of freakers and Marauders in the area. This area also replenishes its supply of crafting materials and Fuel so take as many as needed whenever you visit. Clear the area of enemies if you want then just examine Sarah’s memorial stone for Deacon to have a monologue. After “talking” to Sarah, mission is completed.