300 pounds when the RV is standing still is one thing, but 75 pounds jolting down the road is another. There really isn't much to the ladder, nor what it is fastened to. I would be concerned the mounting hardware and caulk will yield to the jolting, then water could enter the RV, especially at the top of the ladder on the roof. I would play it safe and consider alternatives.
I see people hanging lawn chairs on ladders, which is about all the weight I think is safe.