Bottle of TIME sounds like an artifact.
Also, a proper vehicle doesn't make changing base easy - it becomes your base! I've been debugging just a little, and if you can get the materials you can wind up with some pretty fancy vehicles. Specifically, there's an in-game set of menus that let you modify existing vehicles, allowing you to modify parts (exchange that 1L engine for a 2.5L), remove parts (my wheel is damaged, but luckily there's another bike here with a wheel!), add extra parts (attach an additional fuel tank, add spikes, instal a second engine), and make structural changes (build a roof and walls!). Just looking at it briefly, it should be more than possible to end up with some very creative designs, especially if you're going for a single purpose. A motorbike is very easy, and can include a trunk and such. A truck is also a default vehicle, with significant storage space as well as large gas tanks and a small space inside that's protected from weather.
But if you're designing custom vehicles to deal with the zombie apocalypse... Well, turrets can be mounted, with machineguns, flamethrowers, or plasma rifles. Walls can be added using steel sheets and frameworks, letting you reinforce your vehicle against attack. Roofs can provide protection from the rain. I could imagine a rather small gas engine, or a large electric engine with solar panel being used to power a large square vehicle. This would do very little except provide a 5x5 or larger roof area as well as a bit of trunk space, so that you can fight in the shade casually even in the worst weather!
Similarly, you can remove and add parts relatively quickly, so I might imagine it being possible to attach a few walls and block yourself in, making a temporary "building" where you can read or craft, and then pull down the walls, put them in the trunk, and keep going. I've even asked about how solar panels might work. If you could recharge using them, then not only would your vehicle run clean and infinitely, but you could recharge your radios, flashlights, and perhaps even your bionics by utilizing your vehicle's own solars.
And then there's always the option to take a motorbike, add a bigass engine, spikes on the front, and just mow through the crowds. Depending on how strong things are, you may even be able to crash through walls and perform the ultimate bank heists.