I came up with the same idea for the awning setup when building my old truck and now I have a new and improved setup on my new truck, still slowly working on the new one, not really planing on doing both sides just one side and the rear. My awning can be setup on either side, which is really nice. For the vertical walls, I don't really need or want them on the side awning, just planing on putting walls on the rear part by my doors, so that I have kinda of a changing room. I was going to make them removable with zippers.
To solve the rain issue you guys are discussing, a couple of angled supports makes my awning like a pitched roof, the water all runs off the sides. Of course the water that runs off the back side then runs down the side of my car which is where most of my stuff is, but its just something you have to deal with. You must solve the water collection issue, cause it gets real heavy really quick and don't forget about making it strong enough to withstand some wind, it doesn't take much wind to rip an awning off the side of you car, I've seen it. I have gone through some pretty heavy storms with my setup on my old truck and it worked great, my new one should be better but hasn't seen much fowl weather yet.
My old setup.
My new setup.