None, I carry a telescoping ladder. That way it fits in one of my basement compartments. I don't like the "redneck" look of strapping a stepladder to the roof access ladder.
I like the stability of a regular aluminum ladder with the handy shelf to put stuff on... but an 8' was too long to fit in my bay.
So I hacked 1 1/2 " off the legs and reinstalled the rubber pads with pop rivets ...and voila! Ladder is 7' - 10 1/2 " and fits perfect.
I have a 12.5 foot telescoping ladder from Extend-a-step with over 300 pounds capacity which cost $150. Stores easily in a protective bag in almost any bay. See it here
I believe it is an 8' Step ladder.. Makes a 14.5' extension lader in that mode (yes it is a dual mode) Was not my choice, I inhearated it from my dad.. Dang good ladder though and if I had to choose... Well, 1 foot longer might be better, But truth be told, this one does fine.
We also have the previously mentioned folding ladder. It's important when choosing a ladder for an RV to look at how much weight it is designed to hold. The folding ladder has a limit (200 lbs.?) so is not good for everyone. Also they tend to be a little less stable than an old-fashioned house ladder, which means you really need to hold on to something and learn not to reach too far to the side when on it. When you look at campgrounds, the pad is seldom flat cement and any ladder can be dangerous on uneven surfaces like grass or rocks.
DH and I had a cg neighbor who had one of these and it is now on 'need to get' list. CW has one listed on their catalog website on sale for less than site above. Just saying. We all need to find bargains wherever we can
I also carry one of the folding ladders ( a 7 foot). It does fit in the storage bay nicely, but I am still looking for a better idea. The folding ladder is too unstable for my taste. I would suggest anyone thinking of buying one to first try a demo unit in a store, or perhaps borrow a campground neighbor's for a few minutes. If it fits your needs, go for it. If not, it might be nice to know before you buy.