There are RV-specific bunk ladders, but they aren't the most comfortable on the feet. However, brands like Stromberg Carlson and whatnot make them.
I actually have an extra ladder in my toy hauler that we don't use and will be coming through Columbus tomorrow during the day if you want one cheap.
Otherwise, I chose/preferred to buy a decent wooden bunk bed ladder from Amazon (
link here). That's the one I bought, but it is "unavailable" now. We had to cut the feet down to shorten it up just slightly, but it was solid and well made. But, given the link- I'm betting you can find something else that works.