Ed. I'm just sitting in the garage looking at a small pile of 4x4s. That is to say lumber not Jeeps. I thought of you.
If you need to add 4x4s under your camper anyway, instead of crosswise maybe lengthwise one each side and an on edge 2x4 in the middle. Or better yet a 2x4 framed full footprint. The resulting 3.5" space is what we in the industry call found space. That right there? That's a basement.
Just as you'll learn to love that little stuff door in the bed well, you seem like the kind of guy who could find multiple good uses for found space.