create new uv-mapping on UV channel #3, using Edit UV tool in Front/Side/Top viewports with "Generate New" + "From viewport XY" options. select side polygons and create them UV in side view, select cabin top and hood top and create mapping in top view, etc.
then arrange UV fragments on UV square and render UV in "wireframe atlas" mode. Then save UV atlas render as a template (layout pattern). Create/paint your custom skin and load this skin onto DETAIL2 texture slot with "Manual UV#3" mapping source. Material's adaptation should be set to "truckpaint".
Note, if you use UV#2 for arranging truck template, you have to follow standard truck layout template (found in vehicle/truck/upgrade/materials/matching_pattern.dds). This will allow you truck to be skinned into default paint skin too. your own layout on UV3 can be an additional, so your truck can support original skin template and a custom-layout template.