VELCRO !!!
Spots, not the strips.
You want the slide-out to be able to rip them apart, if you forget to take down the tarp.
Well, you COULD use the strips on the underside of the awning.
As for the "tarp", get a plastic one or, if you are sensitive to these things, a color-matched piece of plastic and attach it with the Velcro (spots or strips) on the underside of your awning and with spots on the side of your slide-out - after you have extended your awning, of course.
Attach to the underside of the awning, first, then match the awning angle (or a slightly more downward angle, for drainage), when you install the velcro on the slide-out.
Be sure to put the SOFT side of the Velcro on the slide-out, so it doesn't tear up the seal, when running the slide in and out.
You will have to use several spots to take the weight of the water, as it runs off the tarp.
After use, just take it off, fold it and put it away.
Remember, you don't have to cover the entire side of the awning or slide-out, just far enough to keep from getting soaked, getting in and out of the RV.
My personal solution?
I was always told, "It doesn't rain, in the Army. It rains ON the Army."
I look at it as an extra bath....