Sagging topper is seldom a problem with the exception of instances where it is pretty much stationery. If you're camping short term or traveling then any pooled water will simply be expelled when you bring in the slide room. If you set up on a permanent or semi permanent site AND there is a lot of rain it might be better to use something to prop up the topper. A support rafter is ok and so would be a beach ball or two placed under the awning. Whatever you put up there, don't forget to remove it prior to closing the slide room or damage may occur.
