We mulled this over extensively a while back trying to accomplish the same. Undermount IMO is just to large an obstacle to deal with, then you'd struggle with location since there is a lot underneath you would restrict access to. Consideration warranted with how to work around running boards (most people have something there).
Ultimately we narrowed it down to two options - both require relatively easy modification:
First - these would need a simple 4-6" segment of a hitch-end welded (or bolted) underneath so when you slide the end of the step into it then the step would be positioned in a useful location. The height can be adjusted by attaching a hitch drop first, followed by attaching the step into the lower piece ***Hitch-drop and step would need to be stored elsewhere until required (just as you are with whatever you're currently using)
Weld this under the frame or to the bottom of your running board:
https://www.amazon.com/CURT-49060-2-...001GMZH5E&th=1
Use a hitch drop as such:
https://www.amazon.com/MAXXHAUL-5065...79&sr=1-6&th=1
Then plug one of these steps in:
https://www.amazon.com/Heininger-305...13&sr=8-8&th=1
https://www.amazon.com/OEDRO-Aluminu...2-1d3da5a5406f
The alternate option would be a portable step like the one linked below - have 2 metal square pipes mounted just under your running board so the top two square protrusions of the step slide in (Would need to drill a hole through both sides to use a cotter pin of sorts to keep them from separating):
https://www.amazon.com/PetRuna-Porta...3&sr=8-23&th=1
We haven't made progress on either one, other than the 'plan' (I'm leaning towards the one which utilizes the dropdown hitch receiver & plug the step into that) - currently we are still satisfied with what we have:
https://www.etrailer.com/Step-Stools...CABEgKWMvD_BwE