On previoius RVs I have mounted various things without issue. Most noteably two full size adult bicycles. I am sure that the weight was in excess of #50. Considering that I could get my #200+ rearend up and down the ladder several times without issue, I would say that long boards or kayaks would not be an issue. The key to anything back there is securing it so it doesn't flop around. The ladder bike rack that I had used 6 different straps to secure the rack and bikes. Look around on the net and see what is out there. Folks are always coming up with new ways to take things along on your rig.
Sarge