You'll want a ladder to get up on the roof if you have a problem up there that occurs on a camping trip. There's a lot more things that can go wrong up there than on a residential roof.
I've had to go up on the roof of our Class C at least three times to fix/check unexpected issues that occured while traveling on trips a long way from home. Of course I use the built-in roof access ladder to safely get up on the roof of our Class C quite often when it's home in storage in the back yard. The reason I say "safely" is - no way am I going to step on/off a step ladder to get onto/off-of the curved and slippery RV roof. The same applies to a ladder propped against the side of the RV roof.