If I understand DOT regulations from when I got my numbers, it is not for a trailer over 10K, it is for a gross combination over 10K. Basically a 3/4 ton diesel with a tandem trailer and maybe 1K on it, and you are doing it for "compensation" (this is a wide open interpretation, it has been construed by DOT for example that hauling horses to a show where you compete for ribbons or trophies, this is "compensation" and you need DOT numbers and follow all the stuff that goes with it, that is time cards, driver medical card, DOT driver qualification files, maintenance files, and all the pre and post trip inspections that go along.) YOU NEED DOT NUMBERS end of story. In Maryland for example to pull over 10K gross comb you need a class A CDL. Fun eh? If you are not doing it for any kind of compensation, you do not need all this junk, but DOT has been trained to figure out in almost any instance some way to say you are operating for compensation.
(If you are in Manassas, maybe you can go down to the scale house at Dumpfries and get your quesiton answered)