I use two Save-A-Step braces under the lowest step on my motorhome. Using them right now, in fact :) Excellent product. Makes stepping in and out of the MH so much better. No need to attach them to the step. Just unscrew to increase the height to what you need. Then store them in a compartment when finished.
I used to have a MH with the ridge on the underside of the step, as you described. You need to check the dimensions listed on the website. I think the product just barely fit between my center ridge and outer ridge. You could also just place a flat piece of wood the fits the whole underside of your step, then place the product against it.
Also, you say your lower step is 13" high. According to the website, the taller one only goes up to 14". So there is a chance, if you have to do some leveling of the trailer, that your lower step would be higher than 14". Then just place a flat piece of wood on the ground under the product to raise it. (Or, of course, the flat piece of wood directly under the step.)