Thanks all for your input. I'll try to answer a few of the Q's
Yes, I would be removing the bunks to run them through the sewing machine.
I am planning to only put reflective material on the 'roof' canvas portion. All the canvas covered would be most efficient, but as others have mentioned, they would be non-removable, so only the critical area would be covered- and colored (orange, green?)
Our front bunk is attached to the slanted front of the camper, so the back bungees have to stretch a bit towards the end of the bunk, which makes it slip off a bit. This allows both sun and RAIN to get under the PUG.