You can add an extra wrap on the tube if you take the material loose from camper. Or you can take the rivets out of the end cap and tighten the spring one revolution and then put a screw back in or a new rivet. Probably need an extra guy or two to help you either way.
We've had the same problem on our main slide, slide topper.
Very cheap fix for this is two cheap inflatable air mattresses. Slide them in on top of the slide head to head and the water will run off and also keep the topper from flapping in strong winds