Agree 100% that the stock ladder is painful for anyone to use, especially in the middle of the night to go to the bathroom!
After extensive on-line searches for a better made replacement ladder, i came to the conclusion that one didn't exist. The one provided by Heartland seems to be the industry standard, unless you want to switch to some type of step stool version with wider steps, but then it is in the way when opened.
For now, we are using an inexpensive pool "noodle float" with the hollow middle. They come in your choice of colors and are much thicker than the pipe insulation, plus they are usually less expensive.
We cut off a slice of the "noodle" just a bit longer than the ladder step is wide in order to have pressure to keep it from moving. We used the large "noodle" as it had a larger diameter hole in the middle so it fits the rectangle steps better on the ladder we have. Then we also secured each end with duct tape to prevent the "noodle' from moving when used. It has worked very well so far, the material gets compressed some but is still much more comfortable than the bare metal.
I have an idea for a permanent installed ladder that will fit when the slide is closed and not cause any issues. It will have a frame to store in so the ladder part can be slid out and locked in place when the steps are needed so you will no longer have to store the ladder on the bunk or the cargo area, it will always be available. A decision I need to make is whether to modify an existing commercially available ladder or build a custom one. If I solve it soon, I will post the pictures and description, still weighing the options.
Best of luck
Keith