Short answer is that you would need to extract the model and texture from the game files, edit the texture and put them back together in Unity.
Longer answer, to rip the files you need Asset Ripper, you can find a link for it in the tutorials section, it is really straight forward to use.
With Asset ripper, you want to extract the vehicles bundle located at "7 Days To Die\Data\Addressables\Standalone\automatic_assets_entities\vehicles.bundle"
Once you have it extracted, you are looking for the files in "vehicles.bundle\ExportedProject\Assets\AssetBundles\Automatic\Entities\Vehicles\Truck"
At this point you can find and edit the ambulanceTruck01.png, this should be the only file you need to edit, the other .png files are for metalics, occlusion, etc. and shouldn't need edited if you just want to change the paint job.
Now you need to put them all back together, for this you need Unity, the game uses version 2021.3.19f1, so you need to use the same. There are some really good tutorials on Unity over in the guides section which I would highly recommend, at minimum the creating and exporting models from Unity ones as that will cover what you need for this. I wont even attempt to write out the instructions as I will not do the tutorial justice.
At this point you have the model extracted and point the .xml to it, and should be good to go.
Alternatively, Ill attach the ambulanceTruck01.png if you want to just jump straight to editing it. I can stick it back together for you once done.
