1. The brackets do NOT scratch the side of the RV. I dado'ed the 1x3 to about a little less than half of its thickness, and notched the tops so the bracket would sit flush with the top of the 1x3. On a couple of the brackets, I had to grind them down enough that the bracket would slide past the rim of the side doors without binding on the side of the RV. Its pretty cool, the side doors open nicely so I can pull stuff in and out. It was raining when I was breaking camp, and it was nice to have a dry spot to put stuff as I was putting everything away.
The only thing I would consider to do differently is to decide if I wanted the entry door to open all the way back to the side of the camper. Right now, it snuggly fits on the top of the tabletop (which holds it nicely as long as you don't need the door to open fully).
I don't have much room in the under storage, so I have to be very efficient with my design. The boards slide nicely on top of my water tank, and the rest of the stuff breaks down so it can be put anywhere.
No, I did NOT put sealant, but now that you reminded me, I will do it, pronto. It is very secure and with the 2 foot wide board attached to the 1" cross pieces via carriage bolts and wingnuts it seems pretty sturdy. If I were to put the 7 gallon water can on top, I would probably opt to put another Tee to split the distance between legs, so it would easily handle the extra weight.