I had asked this same question of Tiffin motorhome customer support when we had our class A Allegro Bus. The reply way "NO" because if you tighten it to the point where the water will run off, it puts too much stress on the slide as it moves to the "out position". This stress can cause the slide to cock vertically when extending, which of course is not good.
I suppose you could use some type of long arm tool to push/place a spacer between the awning and the top of the slide to essentially peak the awning allowing the water to runoff due the the slope.
Days camping (2015)----88 days
Days camping (2016) ---110 days
2016 Wildcat MAXX 28RKX (33' TT), 2007 13' Scamp
2015 Ram Laramie Hemi
30 years RV'ing
12 different RV's