Was stopped once by the KHP while hauling a Ford 4000 with loader on a bumper hitch car trailer, had a bad tail light on the trailer.  He told me either tag the trailer or make sure the weight rating on the pickup tag would cover both.  That is all I know.  My gooseneck trailers are all tagged though.  Been stopped with the same car trailer 2 years ago by KHP and tag for trailer was never mentioned.