I just completed this Campaign. Once I play in Map #1, I recognize the scene with the helicopter and the survivors in the movie I know. Plus, putting the speaker is making loud noise and attract the horde is genius though. After I finish fight the horde, it's so quite in some of the map. Like total silent. Fast forward to Map #5, I was quite confused in this map but I noticed the corpse was holding a remote elevator and I definitely recognize the elevator in the movie called Day of the Dead. It was quite clever to put time of day in this campaign and I freaking love this campaign. Good campaign for Solo or with friends. Highly recommend to play and give it a try.
P.S. Also, I noticed floating First Aid Kit, Molotov, Grenade Launcher in Map #4 and it's quite funny that Boomer stuck in the rock near the elevator. 100% AWESOME CAMPAIGN.
Awesome Campaign