Trailers with roofs, including RVs and third wheels, often have a narrow ladder running up the back. This is supposed to let you scramble up onto the roof to perform maintenance and reach rooftop storage compartments. However, those narrow ladders may be more trouble than they're worth, and in certain circumstances, it may be helpful to remove them and carry a separate ladder with you. Taking them off is not something you should do, by the way; get a maintenance company to detach the ladder and secure the remaining holes so that water doesn't leak in.
Weight Limit and Age
As the Rudy RV Improvement Report notes, those ladders may have weight limits that you don't meet, and it's not easy to tell if the anchors are in good shape. That leads to the possibility of you falling or placing excessive pressure on the anchors if the ladder sags. If you don't meet the weight limits (and really, if no one in your traveling party does except for kids, who you shouldn't allow on the roof anyway), you'd never use the ladder anyway, so why have it sitting there?
Of course, then the question becomes, why worry about it anyway? Can't you just leave it there and not use it? That leads to the next issue: security.
You might not use the ladder at all, but what about vandals and crooks? What about silly kids who are so bored that they've decided to go mess around a bit? Leaving the ladder up means that anyone can try to climb up onto the roof. So not only do you have the weight limit and anchor-stability issues again, but you now have the issue of strangers messing around on your roof. And if it's a rubber roof, that can cause severe damage. You don't know what the person's weight will do to the substrate material at all -- the material can crack in certain circumstances -- and you don't know if their weight will be enough to break caulking seals by placing too much downward pressure on the rubber.
Storage Space and Cost
Really, the two reasons to keep the ladders attached are storage and cost. If you know you meet the weight limits, if you have had the anchors tested and maintained, and if you don't have the storage space to store a separate tall ladder then you may as well leave the attached ladder on the RV. Smaller RVs can be cramped, so it's understandable that you wouldn't want to have to stuff a big ladder inside. The cost of removing the ladder is also an issue because this is not an easy job.
Speak with trailer and RV maintenance companies about ladder testing and removal. It may also be possible to secure the ladder somehow, so others can't climb on it. For more information, contact your local trailer maintenance service.