Don't have a 25.2 but from what I've seen of that slide back there: There is no way a ladder can be mounted back there.
I have one of these collapsible ladders:
6' Compact Folding Step Ladder - GP Logistics SLD-S6 - Ladders - Camping World
Its pretty sturdy. I use it to put the sun screens on the front window when we're camping--I don't think its tall enough to climb on the roof from it though.
There is a taller one that may work:
7' Double-Sided Rung Ladder - GP Logistics SLD-D7 - Ladders - Camping World
not sure though.