If you try to run into the Walkers banging on the door with a vehicle, its possible to push one of them into the warehouse making it impossible to progress the mission since the door will not open until they are all killed. Throwing a Molotov Cocktail at the door may kill it, however.
You can jump on the burned out truck and attract only a couple zombies at a time to make things manageable. Three Infected and 6 Walkers opposed me but I was able to take them down singly and in pairs with this tactic. With only one distracted walker left banging on the door I then looted the area before completing the quest.
Since leaving the truck's vicinity causes the Zombies to lose interest in you this is an ideal way to go get a regular truck from the Lighthouse to drive over Zombies with.
Dead Island Walkthrough - Main Story - Misery Wagon