The RV side connector would be different for the different capacity inlets. For that matter, there could be a few different styles of 50A and 30A RV-side connectors; they aren't specifically standardized by the NEC. Also, many 30A RVs and presumably at least some 50A RVs do not have removable cords, and so have no RV-side connector. (A 30A connector needs two 30A current carrying contacts plus the safety ground; a 50A one needs three 50A current carrying contacts plus the safety ground. Those are significantly different requirements, and so different connectors are used.)
It would be entirely possible to build or have built a 30A cord for a 50A RV, using the appropriate RV side connector and a 30A plug and appropriate (30A) cabling. I'm not sure I would think it worthwhile to get or make one, speaking personally, but it's certainly not an entirely outlandish thing to do.