The speed limit is the maximum allowed speed, right? So there can't be much sympathy for having the drain clogged for illegal speed.
Regarding the time differential, three minutes over 20 miles assumes a clear roadway allowing you to drive at your speed limit the whole way. When the traffic is as tight as you describe I doubt that you could keep up the full speed limit for 400 miles even with no trucks whatsoever. For that reason I am skeptical that the truck speed limit costs you an hour - it sounds like the prevailing speed sans trucks is somewhat lower than your nominal speed limit. The logical resolution to the problem is therefore a reduced speed limit, one that is enforced, which will eliminate the jockeying and unsafe following and other maneuvers.