We have scissors stairs but our truck is too high up for them. I was thinking step stool but didn't take into account the handle thing at the back. Last time out we used a couple of cinder blocks...
They come in different heights. We'd been using a Rubbermaid stool but the dog had issues, probably because he refused to use the handle on the camper. This thing is really stable. Kinda pricey, though.
Edit to add, we use this with the camper's fold down step/bumper which is pretty sturdy, just too high. Not sure if you want to replace the scissor step or just add on to it!