It would be much safer to do it from a ladder. When I service mine I'm always on the ladder.
I don't know if they come with the screw holes already drilled, but if they aren't you could match them up by measuring each one. Kinda tedious, but at least you won't be drilling additional holes in the camper body that way.
Always place some caulking around each screw head and screw threads if you use caulking.
Another way is to use the butyl tape like the factory and screw down evenly while is squishes out. PLastic scraper will get the excess off without scratching. If you ever have to replace it again you won't have to go crazy getting it off.