The problem with adding a ladder is the lack of support in the walls for it. They put extra support in at the factor when one is ordered with a ladder. If you tired to bolt a ladder to the siding it would just pull out the first time you stepped on it. I've heard some people say that some models come with the support whether there's a ladder or not but I really doubt the factory is going to waste any money putting supports in a place they don't need it. It would be a major job to add the additional support after the fact. You'd have to tear out an interior wall to get to it or remove the siding outside which would be pretty much impossible with fiberglass siding without making a real mess and an expensive one at that.
2010 Rockwood Signature Ultra Lite 8315BSS
2003 Chevy Silverado 1/2 Ton Extended Cab
The puma 25RS is a nice trailer (i used to have one) The best thing to do is call Palomino RV and ask them if a ladder can be installed after market. You can walk on the roof of your trailer with or with out a factory installed ladder as is stated in the brochure and owners manual(fully walkable roof). We now have an Outback travel trailer and the specs on it state that it is braced for a rear ladder as standard build.