If you attach a ladder on the side of your trailer you may exceed the max width allowed by most states for trailers on the road. If you install a ladder on your trailer I think you'd have to attach it to one of the studs in the wall so the screws wouldn't pull out. Personally I don't think this is a good idea. Just my two cents. You don't have to get up on the roof that often, so I agree with the others a step ladder is best.
Model X19H Expandable
GMC Sierra Pick up
Been a camper since I was a kid, and that was a long time ago :)
From pup tents with no floors, to tents with floors and flies, to a pop up, to the expandable.