To unlock the Undead Man’s Party Bus achievement you need to upgrade your bus with the three additions available from the Tranzit campaign. Here's a guide:

First, you need to create a turbine. Get a mannequin from the central map, a wing of a toy plane from the payphones and a fan from behind the workbench. When you put them one at a time on the workbench you'll create the turbine.

Grill

After making the turbine, put it next to the door to the right of the bench and when the door is then opened, you'll see the grill inside it. Attach the grill to the front of the bus.

Roof Hatch

Go to the shed near the diner and use the fan to get inside. Here you'll find the roof hatch. Take it back to the bus and go to the middle of the bs. Then look up and jump and you can attach the hatch to the roof.

Ladder

Finally got back to the farm (the very back of it) and you'll see a shed. Use the turbine to get in and you'll find the ladder. Take it back to the bus and attach it to the back.

If a Part is Missing

If you didn't find one of the above parts at the location specified, then you need to look at the terminal. Go to the bank vault area, go left, then go right and follow the path to a locked door. Use the turbine to open the door and you should find your missing part.

When all three parts are attached you will unlock the achievement!