That sunken floor is there for some reason, probably to achieve standing height.
If possible, why not fill it up and make it one flat floor.
Otherwise, covering it with wood would make it easier to cover the sides with wood as well, but you have to be handy for that and have a few tools.
My website describes the conversion of my Ford Transit van into a small RV. I deal with a lot of woodworking, but hope to be quite specific on solar as well.