Many people have reported that under mattress storage is inconvenient to access. Some people just make small shallow drop floors for seldom used items like spare tires, etc., and they are okay with that, but the preferred method for a full "basement" is to have access from the sides and rear.
For a full basement, like you are proposing, I would think that you would want to divide the space for better organization and access. What happens when everything rolls or slides away from your hatch or door? If you can't crawl in or reach something, you would need a hook or pole to fish things out... not convenient.
I think the
Desert Moose is a great example of organized and accessible basement storage.
Also, I'm a little unclear on how you plan to have the cabin sitting on top of the rails. The dimensions you give for the cabin are smaller than the rails. Do you plan on having a curb or "running board" next to the outside wall? Doable, but might promote water settling and creeping in under.
Don't feel the need to compromise your design in order to keep those existing side rails. If you are building a permanently mounted cabin (i.e. not a slide out), the stiffness of a well fastened cabin will more than make up for the structural aspect of the existing top rails. If you are building a slide out, keep them, but a few minutes (maybe an hour) with a cutoff wheel and they could be out of your way. Just saying.
I would think that the best way to organize things would be at least 3 compartments; one across the back with a rearward facing door, and two side by side longitudinally at the front with a door on either side. You should be able to fit an ez-up style canopy, folding bag chairs, folding table, etc. and all of it would be able to be reached.
