I was going to suggest the same thing. My trailer has a ladder, but its pretty low quality and I'm not all that happy with 2 holes in the rubber roof and associated maintenance with that.
My father-in-law has a collapsible ladder that he uses, it seems far more convenient. For the most part I need to get access to the slide roof to clean off debris after trips. With an attached ladder I need to climb the ladder then walk across the roof to get to the slide out. With a collapsible ladder I can lean it directly next to the slide, climb up a couple rungs and blow off the roof. Easy peasy, no extra maintenance and its cheaper than the installed ladder!