If you don't want to waste precious hours on this trophy (because let's accept this dlc wasn't very fun, let alone killing extra few thousand zombies) I found a video on YouTube on how to complete it while you are afk. Just do all the other trophies first so you get as many kills as possible, load up the first level, make the setup in the video and go to sleep. In the morning you will have your final trophy.
Note: Be careful on how you place your vehicle. Monitor it for a while before going completely afk, otherwise you may find out that zombies were not spawning or vehicle position changed and you didn't get the trophy.