Don't have a Keystone, but I would put the body rail up as high as the roof line allows. More slope, the better. Some trailers (like our old Jayco) have a drip rail mounted just above the slide. If you have that, you could remove and put the awning rail where that was. But, I think the best place is just below the roof drip mldg. There is sufficient construction in that area to attach to and, again, gets the most slope out of the awning fabric.
'16 Outdoors Timber Ridge 280RKS
Reese 1700# Trunnion w/ DualCam HP
'03 EXCURSION XLT V10 4.30 Axles