If that's a proper generator outlet, it's a recessed male plug, so the strain is on the wall of the receptical and not on the connections. That said, with a heavy connector like that and the weather seal cover, a quick loop of para-cord or other heavy duty thin cord under the cable and around the hinge of the weather seal cap will take the weight and strain off the cable.
Another solution that works really well is to have a hook above the plug and run a bungie cord from the hook down to the cable to take the weight off the plug.