There is no car rental agency directly across from the RPR. IOA is across from it. International Drive is behind the RPR, and that involves walking across a bridge, which will take you about 15 minutes.
If there is a car rental place in I-drive, I haven't seen it. Maybe the locals know of one.
If I was staying onsite and wanted a car for a day or two, I'd look into Enterprise with their "Well Pick You Up" slogan.