FATIGUED FLT CREW OF E145 EXITED THE SNOW OBSCURED PAVED RAMP AREA IN EVV DURING NIGHTTIME ARR AT EVV; AN UNCTLED ARPT AT THE TIME OF ARR.

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FLT XXX ORD TO ZZZ. AFTER LNDG ON RWY 22; TURNED N ON RWY 36; L ON TXWY F. CONSULTING THE ARPT DIAGRAM; THE ENTIRE TIME WE WERE GOING TO TURN ON TXWY A TO GET TO RAMP; WE PASSED TXWY A AND HELD SHORT ON TXWY C. LOOKING AT THE DIAGRAM WE DECIDED THE SAFEST COURSE OF ACTION WAS TO PROCEED TO THE DEPICTED TXWY C RAMP AREA AND REVERSE COURSE. THE DEPICTED AREA WAS MUCH SMALLER THAN EXPECTED; BUT THERE STILL SEEMED TO BE ENOUGH ROOM TO TURN AROUND. WE STARTED A R 180 DEG; AND I SAID STOP; BECAUSE OF SNOW OBSCURING THE EDGE OF THE TXWY. I WAS NOT COMFORTABLE CONTINUING. THEN BECAUSE OF WHAT COULD ONLY BE EXTREME FATIGUE AND EMOTIONAL STRESS; THE CAPT BRIEFLY USED REVERSE TO AID IN THE TURN. I WAS NOT CONSULTED; AND DO NOT KNOW IF THE CAPT EVEN KNEW WHAT WAS GOING ON AROUND HIM. I STARTED TO ASK THAT WE STOP FOR HELP AS I HAD SUGGESTED BEFORE. AFTER ATTEMPTING A L TURN; WE BEGAN TO SUSPECT THE L MAIN MAY BE CAUGHT IN SNOW OR ICE. IT WAS NOT APPARENT THAT THE L MAIN WENT OFF THE TXWY. WE STOPPED AND REQUESTED HELP; AND I CALLED MAINT CTL AND MAINT ON CELL. THE PRIMARY CONTRIBUTING FACTORS TO THIS INCIDENT WERE THE CAPT'S EXTREME FATIGUE AND MY FAILURE TO RECOGNIZE THE FATIGUE IMMEDIATELY; BECAUSE OF MY OWN FATIGUE. ALSO; THE POOR ARPT DIAGRAM; POOR ARPT LIGHTING ON THAT PORTION OF THE RAMP; THE ARPT BEING UNCTLED AT THAT TIME OF NIGHT; SEVERAL GA ACFT IN PATTERN THAT DID NOT DESIGNATE WHAT RWY THEY INTENDED TO USE. BUT; MOST IMPORTANTLY; WAS THE CAPT'S EXTREME FATIGUE.