FWIW, for the life of me I can't see a real and logical reason - worth worrying about - not to have solid built-in ladder access to your rig's roof provided by it's manufacturer. I wonder if the main reason to leave it off is cost savings by the manufacturers?
Personnally, I wouldn't buy an RV without a built-in ladder that also looked to be solidly mounted. We had a van-based RV once withouut a roof ladder, but of course I could reach all things up on the roof by leaning over safely from a step ladder placed right beside the van's wall ... but not having to try to get on/off the step ladder to/from the roof.
I "think" that some RV manufacturers may be trying to provide roof access through a large vent in the ceiling. For instance, certain Earthroamer and Lazy Daze models may include a large vent for this - not sure though. This approach I can kind of buy into.... assuming one is willing to accept the additional risk over time of leaks from a large vent in the ceiling.