Anytime any asks about installing a ladder on an rv without one, I always suggest not doing it, for a couple of reasons...
1) it's expensive for what it is,
2) it is only a one trick pony, can't be used for anything else,
3) if not installed by the factory, the backing/support is probably not there
4) AND, they didn't install one because the roof may not be designed to support anyone up there
5) finally, get a collapsible ladder that can be used all around your rv and at home if need be... and get one at least 3 feet longer than your rv is tall...
and pay attention to number 4 above