Okay, an update. If you're anywhere around New England, you probably know the weather has been atrocious over the last few weeks. Since we haven't been able to be outside to work on the trailer much (save to repair the twine that keep snapping, causing our tarp to blow off every other day - which we finally fixed by using sisal rope. No more snapping ties - just in time for the rain/sleet/ice/snow that's happened in the last 24 hours). So I've spent that time getting frustrated with Fusion 360 while I do my best to make a render of our plan.
I have to say, doing a 3D model - as frustrating as the learning curve is - has been a spectacular idea. There were ideas in my head that, when I saw them in the render, I discovered there was no way it could work. So I finally think I've got it nailed down. I'm updating now, while it's fresh in my head, because Fusion doesn't let you put notes into your renders, and this'll be a good thing to refresh my memory of what my thinking was in the decisions I've made. Because I know I'll forget later, and ask myself "Why the hell did I do THAT‽?" (thank you ADHD) This'll be a long one, and truly it's mostly so I don't forget what I'm doing (i.e. selfishly for me), so you don't have to read the whole thing if you don't want to.
1. Storage area

So the idea is that once we clean off the trailer frame, get it rustproofed and all, we will "sandwich" the frame above and below with ¼" of material. Not 100% sure what material we will use yet - we're thinking the undercarriage could be that corrugated sign material, or even just a big sheet of plastic or something lightweight. We plan to foam between the frame supports, so the floor will actually have a good 4" of insulation. In the photo, you see truss supports, which I plan to make from 2x2 balsam wood (if we can find it). Each support is directly atop a steel beam of the trailer frame. The water tank is between the wheel wells - the idea is, if the kids decide to ...I dunno, visit Alaska or Colorado, the water tank will be insulated and won't freeze. These truss supports will be the floor supports for the cabin area, as also act as "inner walls" for underfloor storage. The voids just in front of the wheel wells will house pull-out stairs to get in and out of the cabin. You'll also note they only go so far back - that's where the kitchen will be (I'll explain more in a moment). The truss idea keeps the floor "joists" lightweight (they're essentially holey 2x10s), and because they are "holey" there's plenty of space to run electric and water lines without drilling into anything.
2. "Framing"

Here you see the framing, as well as the addition of the cabin subfloor above the truss supports. Because this was originally a popup camper, I'm keeping the popup mechanism so it'll travel low, but be pretty tall when camped. But since the actual cabin floor won't be directly atop the frame anymore, we're going to use supported PVC pipe to run the cables through so they go where they need to go without ever getting tangled up in something in storage. I've decided to also put some dowels in the 2x2 extensions supports to help hold the walls in place. Connecticut requires trailers to have doors attached to actual framing material, so I've added that, as well.
3. Cabin Walls

Here is the foam we're attaching to the framing. there's a 1.5" layer on the inside, and a ½" layer on the outside (to cover the trailer frame. What you don't see are the holes I'm cutting to access the stairs under the door and storage area, but there will be doors so you can open them up and access all of that.
4. Outer Shell

This is the outer shell. It's made of 1" foam, and will be attached to the extensions so it can "pop up".This photo is what it looks like when traveling. Of course the door isn't there - we're still deciding what the best thing for the door will be like. We've got it boiled down to two ideas: either it'll be a sliding thing (that slides up and down with the outer shell); or it'll fold in half and we use the pins from the door hinge to hold it up in place. We keep going back and forth on which is the better option. We'll figure it out, though.
5. Kitchen

This is from the back side, with the outer shell popped up. The back is a hatch that will open to the kitchen. This is actually the part that has taken me the better part of 2 weeks to figure out! The kitchen hatch will use the upper cabinets to rest on and stay open. Before we PMF the foam, we're planning on embedding strong magnets into the foam at the top edge of the hatch. Then when the camper is popped open, you can roll down a piece of waterproof fabric that will magnetically attach to the edge of the hatch to keep rain from getting inside through the top (when it's closed).
The inside back wall, the thing with the little circles running up the center? That's a bit complicated. It's a wall that slides up and down to separate the cabin from the outside. But because it has to be
inside when traveling, the walls are about 2.5" too short on either side (and bugs can get in). So instead of having a rooftop fan for air circulation, I'm going to buy three 200mm computer fans and line them up in a frame. I'll sandwich them with window screen, as well. The strip will just be inside the cabin while traveling, but when camping, you can push the walls to either side to close up those holes, and slip the fan strip in the middle to lock them in place. Plug it in, and insta-ventilation. This will also free up the entire roof for solar panels (and we're planning to try out a couple of 3D printed wind-catchers, as well).
6. Electrical equipment and propane

So this is from the front, and I haven't rendered the wall that goes here, but there IS a wall that goes here. It's going to be split in half. The lower half will house the propane tank(s). My husband insists that this lower area be well-separated from the cabin, and the front of it will have ventilation in case of any leaks, and it will be accessed from the outside. The upper half will actually be accessed from the inside, and house all of the electrical stuff and batteries for the solar/wind array.
What's not pictured is the collapsible table and bench seats on the inside. The table will actually be a foldable type that can be taken outside to sit around, or stored underneath. There will be two bench seats that will raise into 2 bunk beds so it will sleep 4 adults comfortably, and have a nice, wide area in the center for getting dressed or getting up in the middle of the night to go to the bathroom without waking everyone else up. I'm also debating on making a composting toilet and a collapsible shower - but my son in insisted neither is necessary. I just want to make them anyway to see if I can. We'll see how it goes!
And that's it - that's the plan. The thing is 12.5' long and 84" wide. When collapsed, it's roughly the same height as my CR-V - about 5 feet tall (give or take), and just a little bit wider. When the popup is...popped up, the interior cabin height is ~8' (which was by accident, not design, but I'm not gonna fault it as my son and his friends are tall anyway, and they did ay they wanted to be able to stand up in it) When the kitchen hatch is open, the cook has about 6.5' of vertical space to move around in...provided they park on level ground. I'm trying to keep as much weight as I can atop the axle towards the front, so I'm DIYing the fridge (I know - I said I was trying crazy stuff!), and they want a pull-out kitchen drawer with a camp stove built into it (that they can attach the propane tank to). Possibly a toaster oven. But they're not looking for a pro-level kitchen or anything - just something they can make coffee on, maybe grill cheese sandwiches or heat up some soup, and maybe bake up some bagel bites.