I don't know about trailers, but I do recall that the Maine Coast Railroad had a boxcar which was painted bright yellow and had information about the passenger train rides on it. They used to park it on the railroad bridge over Rt 1 in Brunswick (on the Lower Road) as advertising. Somebody (State or town I don't know) made them unpark it, probably because of the billboard laws.
Any billboard-like signs you see nowadays are probably grandfathered. Boothbay Railway Village used to have one in Wiscasset, just north of the Wiscasset Motor Lodge. When the owners of the land changed the sign went away and they had to get the standard directional signs.