AN ATX SMA GOT LOST WHILE TAXIING IN THE FOG AT SDF.

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AFTER LNDG AT SDF WITH AN EVER DECREASING RVR; I ATTEMPTED TO TAXI TO THE FBO. AT THE BEGINNING OF THE APCH; THE RVR FOR RWY 19 WAS 2500 FT. THE TWR RPTED 1400 FT WHEN I WAS INSIDE THE MARKER. HOWEVER; I FOUND IT TO BE A GOOD 1/2 MI UPON REACHING 300 FT. ON THE ROLLOUT; HOWEVER; RVR DECREASED RAPIDLY; MAKING TAXIING DIFFICULT. I WAS INSTRUCTED TO TAXI VIA TXWY G TO THE RAMP; XING RWY 29 ENRTE. APCHING TO CROSS RWY 29; YOU LITERALLY COULD NOT SEE THE TXWY ON THE OTHER SIDE. TWR INFORMED I COULD TAXI STRAIGHT ACROSS TO THE RAMP. AFTER XING RWY 29; I CONTINUED TO TAXI ON WHAT I THOUGHT WAS 'G.' JUST PAST THE FIRE HOUSE; TXWY G BRANCHES OFF TO THE R. I SAW THE FIRE HOUSE GO BY BUT I DID NOT SEE THE TXWY BRANCH TO THE R. I CONTINUED FOLLOWING THE CTRLINE WHICH WAS NOW TXWY H. I KNEW I WAS LOST WHEN I GOT TO RWY 19. I STOPPED JUST SHORT OF 19 AND QUICKLY DID A 180. THE TWR HAD ISSUED A TKOF CLRNC TO ANOTHER ACFT AT THE SAME TIME I NOTICED I WAS IN THE WRONG PLACE. TWR CANCELED THE OTHER ACFT'S TKOF CLRNC. I WAS TURNING AROUND. FURTHER TAXI WAS WITHOUT INCIDENT. THE INCIDENT WAS CAUSED BY A VERY LOW VISIBILITY; COUPLED WITH THE FORK IN THE ROAD AND THE PLT'S UNFAMILIARITY WITH THE ARPT. I BELIEVE THIS PARTICULAR INCIDENT COULD HAVE BEEN AVOIDED BY THE PLT ASKING FOR HELP SOONER AND CLOSER ADHERENCE TO THE ARPT DIAGRAM. THE CTLR DID AN EXCELLENT JOB ISSUING INSTRUCTIONS AND A CONFLICT REALLY NEVER AROSE. THE TWR PROBABLY COULD NOT EVEN SEE I HAD TAKEN A WRONG TURN.