The trucks are invisible at long distances if you're using a large object view distance with normal object detail setting. Not sure if this is because the vehicles don't have good lower LOD models or if the engine just decides to cull them at that distance when using normal object detail setting.

I honestly think that the geometry of vehicles and other gameplay relevant entities should render out to the maximum object view distance regardless of the object detail setting.

Standard object setting
http://cloud-2.steampowered.com/ugc/558759332659832347/FA3937E567A4599CE6297BBD23B3A093317EEAA1/

Ultra object setting
http://cloud-4.steampowered.com/ugc/558759332659835852/52E20EE6D5E7573BCA2F1211BF25997D86D71D62/