Perhaps a raised false floor under the rear, passenger side, dinette seat. You could get a little more length by adding a hole on the forward wall to take advantage of a little space in the wheel well, but there is not much to be gained. A few Velcro tie downs would secure your stuff. There should plenty of room for other things. This won’t work if you have an exterior hatch.
Under the same seat you could make a small storage area along the kick panel that defines the inner side of that space. Add a new panel parallel to the existing kick panel with a removable lid. Put a few fake wires or pipes on it, and into it, and it may look like part of the utility system.
You could make the front of the cabinet above the large rear dinette window removable with a false rear wall.
Making the wheel well panels behind the doors removable via Velcro strips gives enough space for hand toys.