In BC Canada if your truck is obviously overloaded, such as when the headlights are pointed at the moon, an officer can have you weighed and ticket you for being over your gvwr. I'm not aware of any other jurisdiction that uses gvwr as a legal number. In Alberta Canada if an officer thought you were grossly overloaded he would likely ticket you based on axle or tire rating. I've never heard of an RVer being ticketed for being overloaded.
As others have pointed out a 20,000 lb 5ver will most likely require a dually to carry its pin weight.