We all seem to forget that your GPS is only as good as the map data provided to it. In the OPs example, whoever supplied the map data to the company that supplies Garmin, made a mistake or didn't update their data. The net result was what the OP presented. Don't blame the GPS it's only plotting a route based on the data provided.
As to others who think that the car route is better/shorter, forget that RV GPSs generally route RVs on truck routes because of the weight/height of the RV. Weight limits on roads can put you on some strange routes. RVs may be exempt from those limits in some states but the GPS doesn't know that.
Do us all a favor and report mapping errors to the GPS manufacturers - they really do make corrections.