Chris, you have the right way of going about getting the 'extra' room you'll need. Stand-up space is usually only where you stand. With an outer wall, you usually don't stand next to it. The folding Compact design is just for that walking/standing space. So to have that extra space when you lift anything bigger.
With your drawing, you are wanting to keep the utility rails on the trailer and make the camper more of a slide-on? If you decide to keep the rails, but not build a slide-on, you could make storage compartments on the side of the rails under the camper to 'complete' the look of the camper. You could also make that space accessible from the inside as well as outside.