If you are a weekend camper you probably won't need one. If you need to get to a high spot on your RV you simply wait until you get back home and pull your ladder out of your garage.
Snowbird or Full time and they are almost a must have. Washing, waxing, cleaning the roof, getting up on the roof, etc. We have to "maintain and clean" our RV's "while on the road".
The ladders on the back of MH's are not that sturdy to be using often. And climbing up the ladder on the back to the roof is not going to help you wash the windows on the side! :B
Also depends on what type of RV you have. You can't reach the windows in a MH from standing on the ground.
Best ladder carrying I ever saw was a full body paint MH that the owner had the ladder painted with the same body paint so when it was attached to the back of the MH it blended right in with the graphics on the back of the MH!