My (proposed) ramp is going to be used only for wife's chair to go in and out, so I'm leaning towards something "grabby" so if it's wet it doesn't turn into a water slide for her

Maybe bumpers/curbs on the sides so it's at least a straight slide and not a dropoff?
That probably means either the rough cheap expanded metal or some sort of indoor-outdoor carpet. Thing with the carpet is, I know she's gonna wear tracks in it very quickly, so it'd have to be something easily replaceable. I'd was going to try to build in the option of using the ramp as a porch, but realistically can't see the point of sitting a few inches higher on a platform instead of just hanging out on the ground either at the bottom of the ramp or around side of the trailer.