Really depends on the length of time you're on the road. If you're just doing weekend to week trips there is no real need to bother with one. Once you get back to your S&B you can use the ladder there to do any needed work. If you just have to have one you can always borrow it from someone in the campground. Great way to meet people and everyone understands you just can't carry everything.
We lived on the road for about 6 months per year. I had to have a ladder, and I used it often. From sealing joints to waxing that top foot of the rig. For me a five foot ladder was perfect. Enough height, light weight and fairly compact.