I do not, partly because I don't have a hitch-mount cargo carrier (aside from a bicycle rack), and partly because my RV has a built-in permanent tank.
However, I think it a not very wise idea to carry propane outside of the limits of the structural framework of the vehicle. If I'm not mistaken, DOT regulations prohibit mounting propane tanks such that they extend over the bumper or under the frame, though I don't know if that applies to portable cylinders or only to permanent tanks that are "part" of the vehicle proper. In any case, the potential for bad things in a collision should be pretty obvious, even for an otherwise comparatively minor one, and it's not a chance I care to take.