Getting stopped by any cop is a pita.
And IMHO I don't think there is a cop on the road that knows ALL the states different requirements for all the different types of vehicles/trailers.
And if he is coming up from behind you he doesn't 'know' which state you are from to make that decision whether to stop you or not. He's stopping you because you have no plate.
With all that is going on nowadays with police stops? That is why I 'did' purchase a plate that was not necessary for my TT to AVOID the whole scenario. Why do you think DMV offers that option? :W