Having installed a slide topper awning from scratch, I can tell you that the camper mounted edge of the fabric is supposed to be higher than the roller. Higher by as much as three inches if I recall correctly.
And the roller should be snug so that there is tension on the awning. A properly installed and properly operating slide topper should shed water.
Not all manufacturers make proper room to allow a topper to be properly mounted!
And not all dealers install them correctly.
Tensioning the topper will probably require removing the awning to start with. . .it will be a two person job.