Porting Meshes to Trailmakers

Open the mesh file in blender, select all the meshes and press N on your keyboard, this will popout a side menu with the option of MatCombiner, whilst the meshes are still selected click on MatCombiner then then make sure you atlas is set to automatic and then click on Save Atlas to and save the atlas image to a location that you'll remember; be careful not to deselect the meshes yet.

Now whilst the meshes are still selected go to File > Export > Wavefront Obj and save the mesh file.