The stock, attached ladder has two disadvantages: it's straight up and the transition from ladder to roof is less than ideal.
Extension ladder issues I can think of: 1) What are you setting it on? In my case, that would be uneven ground, which would require a lot of blocks to make a safe footing. If you're on pavement, not an issue, probably. 2) How sturdy is the roof molding? Is it strong enough to take your weight as you go up? And will the pressure from the ladder disturb the screws/sealant? I've got an Itasca Navion on which I spent a couple of days resetting the rear roof molding (started out as a backup-camera replacement, until I discovered the holes in the adhesive flashing under the molding in one spot, which led to removing it all, so I could reflash the whole area...you know how that goes). No way would I lean a ladder against it after the work I put into getting it right.
In general, an extension ladder should extend past the surface onto which you'll be stepping by at least three steps, so you can grip the ladder as you make the transition.
And, of course, make sure your ladder is at the correct angle: stand with your foot about even with the bottom of the ladder, reach out an arm without twisting your torso, and grip the rung that's closest to level with your shoulder. Your arm should be straight. If not, correct the ladder's placement.
