I don't know of any TM-mapped roads worse than KY 199. But a thorough look for one could start in Puerto Rico, which has some really awful rural roads that may have route numbers.
Hawaii has some in that category, but Hawaii DOT made a point of fobbing them all off on the counties (whose routes we don't map) when it split a combined state/county route system into separate state and county systems ca. 1968.