This is kind of true.  Most big rigs are a base 24v system.  The alternator, starter, engine(ecu and sensors) and many other things are 24v parts.  They do have a 24v to 12v dc-dc converter (which is a lot less expensive than going 12v to 24v) for all the standard stuff such as cigarette lighter power outlets and lights (some even have 24v lighting systems).  I state this based on my own experience with my father's rig, which is several years old.  It wouldn't surprise me if the newer rigs run more off the 12v system, since the price of 24v to 12v converters has dropped a lot in recent years, but they still must use a 24v alternator to charge a 24v system.