There are not a lot of options that come to mind. You may have some amount of two inch tube exposed that comes out of the motorhome receiver hitch going to your car coupler. Without getting too complicated it would seem that you could get several U bolts (muffler clamps) that could go around some PVC pipe and also clamp onto the receiver tube with a couple of pieces of flat bar stock between the Ubolts on the opposite side of the square tube from the side the pipe is on. Having the pipe be maybe 3ft high starting from ground level would add some stability and could work. You would drop the flagpole (height within the limits of the strength of this mount) into the PVC pipe.
Later Ed