My motorhome will likely have a gate across the opening before an unready toddler visits. A gate is probably a good idea for older folks too, keep them from accidentally steep off into the hole.
The safest way to deal with the attraction and the hazard of stairs, for a toddler, might be to teach the child how to handle stairs.
My toddler grandson is stair aware, has been climbing stairs since before his first birthday, even before he could walk, so it was necessary to teach him how to crawl back down. A few months short of two, he now walks up and down some stairs, crawls others if they seem like too much. RV stairs, he would quickly figure out he needs to crawl or get some help.
In just a few hours you may learn about some of the other attractive hazards a RV interior offers to an inquisitive toddler.