I use a telescoping multi-position ladder to access the roof from the side that in its stepladder configuration is higher than the RV roof. This ladder has wider legs at the base than the top and is very stable. This stepladder, along with another 10' stepladder and a telescoping aluminum plank also serve as a platform to work on the front of the cabover.
While traveling we carry a folding 6' stepladder that folds to 4.5" x 4.5" x 6'. This fits in the pass-through compartment in the back and is used for any inspection, cleaning windshield, etc. If I need to get on the roof on the road then I will use the rear, built in ladder.