Renchtal is a medium sized service station North-West of the small town of Appenweier at the A5 between Offenburg and Baden-Baden. It is easy here to catch lifts north (Frankfurt) and south (Basel) and it serves an excellent starting point from Strasbourg and Offenburg. It's very hard to get to Strasbourg from here though.
If heading into Germany from Strasbourg you may be quicker if you take the cheap train to Appenweier and walk ~25min to the service station. From the station, head west on "Urloffer Weg" and turn right on "Hauptstraße". Turn left right after you crossed the small creek/canal, follow the canal and turn right (after ~500m) on the road that leads to some farm building. Follow this trail all the way to the spot where a bridge crosses the highway. The service station is ~100m north of this point.
You may be much quicker on the motorway from Offenburg if you travel by bus/train to this service station than if you start thumbing in Offenburg. Either take the train to Appenweier and follow the trail described above. Or take a bus from Offenburg to "Urloffen Rose" (bus 7136 direction Bühl or Heimschule Lender, or bus 7136 direction Kehl). From the stop head south-west until the Ford car seller and turn right (direction Legelshurst) and walk out of the village until you cross the motorway right where the service station is situated (10 min walk from the bus stop).