I played finally this campaign and although is good, we found several problems while playing online, so we would like you to fix/improve them for the next version:
First, the inmense lag that it has the map. I noticed you spawn zombies both where are the players, and where not. You should avoid that, and having a dynamic system which spawns them depending of where are the players. Also, make the props with prop_multiplayer_physics or just prop_static. For the ragdolls, follow this tutorial: http://dods.gamebanana.com/tuts/8484 and make then not collide with players (debris) or just disable their motion.
Second, there's no ammo on the first map, only at the safe rooms. If you ran out of ammo, you will die soon, since there aren't medkits neither. You should add some boxes and healvials around. Same goes with the second map, but for the healing items.
At Expert difficulty, the fast zombies will likely to insta kill you, since they attack too fast. Either lower their damage, or use the normal zombies.
The zombies doesn't seem to use squads, or a npc_enemyfinder, so sometimes you find them idle, being able to get past along. You also can find some npcs getting stuck on some places.
At the second map, you should add more lights to the plaza terminal, since if all the players use the flashlight, the FPS will go down quickly. Also, indicate somehow (either with a light, or a camera, or something), that the players must go into the station, and that a Tank has appeared.
About the Tank, it seems that it doesn't animate when attacking, you should check their animations and making them play through their QC file.
You should add/make some kind of visible texture to indicate the players that they can't go through some places, instead of the invisible wall.
At the first level, you only can access the second staircase after finishing the horde, which is stupid since is an invisible wall. You should change it for an event (like for example, a small fire that takes time to burn a barricade, that could work).
And that's all. You make very interesting maps, and we hope we continue seeing your work. Thank you for reading this.