If you told me you were in S.C. on 95 south and at mile marker 51 I'd find you with no problem. It is obvious the GSRA has some uneducated folks in that office. Since OP was at mile marker 51 he was 51 miles from the state line going south.
Okay Good Sam Roadside Assist pass this along to all your dispatchers so that they have a clue:
On one- or two-digit interstates, the numbering always begins at the south state line (for north-south interstates) or the west state line (for east-west interstates). Thus, Mile Marker 1 on I-65 is just north of the Kentucky state line and Mile Marker 261 is near I-65’s northern terminus at I-90 in northwest Indiana. Likewise, Mile Marker 1 on I-70 is just east of the Illinois state line and Mile Marker 156 is just west of the Ohio state line. Mile marker numbers always get larger as you travel north or east on one- or two-digit interstates.
So this would have put the OP 51 mile north of the S.C. and Georgia state line. Somewhere near Brunswick, GA. Daaah!