just some idle thoughts .... its about 12 years since I went through my design exercise, so had to resurrect it from memory and technology has improved/changed. One of my constraints was the cost of solar panels so I built my vacuum insulated fridge. Another was inefficiency of aircon (COP has greatly improved) - I calculated for, but didn't install. Another was battery, I started with AGM and diesel cooking, now have lots of solar, Li, and induction cooking. Finding "insulated" curtain material was a challenge in Aus at the time. Different foam densities for seat squabs and backs was as important as squab and back angles.
In the compromise between insulation and internal volume I used walls 20mm, roof and floor 50mm. Easier to grow upwards (to a limit) than lengthways or sideways. For a 4x2x2.1m external box the difference between all 20mm and all 50mm is about 7% less internal space, about 1m3. If all100mm its about 18%, 3m3 less internal space. It doesn't sound/look a lot on paper but it "feels" a lot when inside. Space is subjective as well as objective. It also eventually gets down to a loss of 25mm (for example) in the shower/loo is lots. I've forgotten precise dimensions, my shower/loo is about 1.2 x 0.75m ... relatively large, I considered it a necessary comfort/convenience for the countries we would travel through where hygiene can come at a premium,
One consideration around the subjective nature of "space" was the only thing higher than bench top is the shower/loo.
There's also subtleties like depth of bench, thus drawer length, and the distance between bench and whatever is opposite so as to be able to insert and remove drawers. I have about 10mm with a 660mm deep bench and 650mm fully opening drawer slides. Fridge was built with drawers inside instead of shelves due to depth, and contents packing density. Bench top is 2m x 660mm.
The same discussion about external wall thickness occurs with drawers. The difference in drawer volume between 10mm and 3mm (I used Dibond - Al faced polyethylene) wall thickness is significant. I hated losing drawer volume to drawer slides but too lazy to inset them - there's a law of diminishing returns, and build time.
I notice mention of running costs. Just checking, though I should assume you know ... once installed the off-grid cost of electricity from solar is $0. To digress, the zero marginal operating cost of solar and wind is why renewables are always consumed first in our electricity market and ultimately create a permanent downward pressure on market prices that is unachievable with other energy sources. Our RV design constraint is how much solar can be installed on the roof. My first panels were about 16% efficient, more recent 22%. When I cover the roof that's a big difference in available kW. About 38% more. In detail I have four panels with synchronised individual mppt regulators.
The $0 operating cost changes the energy efficiency calculations. When I designed my truck it was more cost effective to import vacuum panels for fridge than buy solar panels. Now its more cost effective to cover the roof with solar. I originally put a lot of effort into energy balances, including the heat balance for aircon. Now I suspect the additional cost of an extra solar panel becomes part of the trade off between wall insulation, aircon size, internal space, and battery capacity. In Aus the Mitsubishi Heavy Industries split aircon, 2.2kW, seems to still provide best COP. Turndown ratio means minimum input power about 300W. I run into weight constraints (4.5mt).
Also an aside - In a normal house situation energy efficiency (and emissions) changes with solar. Its cheaper and more effective to add rooftop solar than double glazing. Energy efficiency of double glazing does nothing to reduce emissions from generation if there aren't any in the first place.
My brief diversion into flexible panels (to save weight) was aborted when panels failed. Importantly for heat load, the increased roof temperature due to different mounting (different to glass panels) was sufficient to bow the roof by about 10mm in the centre. Internal temperature of roof noticeably higher. I kept a couple of flexible panels as portable, to deploy when required (occasionally parked under a tree). I hate setup time.
I think I mentioned leaving the ventilation out of the door when I rebuilt it (around same panel type as wall). I thought I didn't need it because of no gas. In practice the lack of it creates a "closeness" in some conditions. Comfort is more than temperature.
The challenge for solar is not the sunny days. Its the rainy days. Backup is standard engine alternator. Rarely used, we instead progressively reduce our energy usage. Step one is usually cook meals that require less cooking time. And fewer cups of coffee. In Aus its not hard, we have a good climate for self contained RV. Best solar we've had is north of the Arctic Circle in summer - a few amps for lots of hours, the battery was almost redundant.
While I'm here. We like washing dishes ....... ! Water depth seems important. A smaller sink has deeper water for the same volume. Insulating the sink keep the water hotter longer. Part of water efficiency. A month in the Simpson Desert is relatively easy. I also operate the water system at lower pressure, about 6psi. In the days when solar was at a premium it didn't make sense to me to operate pumps at 35psi for little extra flow - as an aside, our city wide reticulated water is horribly energy inefficient.
Your design will be very different to mine, I can only describe mine and pass on the thought processes behind it. There usually isn't right or wrong, just different. I hope this helps / is useful.
Something else you probably know, but just in case. I have 9 external hatches and 9 internal. Each has a door, and a frame. The doors are the foam sandwich cut out. Each frame has four pieces of Aluminium Zed. I would have preferred fibreglass pultrusions but no-one could spell it 12 years ago. Each door has four pieces of Aluminium channel - hollow square slit in half. That's 144 pieces of Al. Plus 30 compression latches, plus 18 continuous hinges, plus 72 pieces of foam seal. It all adds up to build time.
And as an engineer you'll understand that things require maintenance, aim to have everything removable for maintenance with minimal effort. Also can be isolated. By way of illustration. Nothing worse than crawling underneath on stony ground in 40 degree heat trying to remove a tank that wasn't designed to be removed to fix a water leak, when an extra valve could have isolated a section until more clement conditions prevail. System redundancy is good.
I started with a need for container shipping. I have a "drop top". Always up, except for the couple of times in its life it needed to be down. I don't have a pass through. SkiFreak is better at explaining the compromises involved in solid sided pop top and pass through. On half a dozen occasions we've found our height (3.2m) has prevented us negotiating a track due to overhanging/angled trees.
