Depending on how well your toppers were installed, you may want supports under them even during summer rains. This is especially true for the large dining room slides. Otherwise, the rain will pool on them and you'll end up with algae stains or noisy toppers when the wind blows. Our three small toppers are tight and don't sag, but the main slide did, so I made a PVC support similar to what Dan showed. Our first outing, years ago, I had rain water get a surface skin of ice on it. About pooped my pants when the sheet of ice slid off and hit the ground when I was pulling the slide in. Luckily, no damage done. That's when I learned about supports.
At rallies, I've seen new Landmarks with toppers that looked like poorly hung hammocks on the smaller slides. Not good.