Cardboard Banker Boxes
I just bought an 8 pack of these at Costco and they are very strong and very light with a nice tight fitting lid. They come in a flat pack and when you need a new storage box , you just fold it out and presto you end up with a nice box 12"wide 10" high and 16" long. I am going to put a few under my under bed storage compartment alongside the hydraulic rams.
I will cut the height down to fit this spot if I have to. The slide is in right now so I am not sure. These boxes fit inside the cupboards over the rear picture window as well as a few other places and they will be great on the closet floor. They are made to store legal documents but I think they are a good storage solution for RVs as well.