I am currently at the beginning of the same project based on a 2007 NPS. My time has to be shared with other demands and hence progress is slow.
My design started out with roof height as a key criterion and I wanted it to be able to go into a container. My solution was essentially a shoe box with the long sided overlapping lid being raised by linear actuators. This is still on the books as I have all the stuff to do it. We have taken a 4wd overseas before and have now decided that we have probably done all our big o/s trips and in future we will source a vehicle in the destination country as needed. This has resulted in us probably backing off to a rigid wall box along the lines of your design.
I am keen on modern technology but in my build I am using established commonly available components. First it is easier to find replacements when needed and the cost is far lower. I agree it is nice to push the margins and if you have the money then great but make sure you consider back country repairs.
I have used Redarc equipment for many years and whilst it is not leading edge it has proven to be robust and flexible in use. The company also really stands by their products and are very helpful with advice. The replaced an intermittent dual battery solenoid outside the warranty period because it had failed within warranty but I only returned from Central Asia much later. I am using several of their bits of gear in the new build based on my experience.
I would think carefully about using 24v for the house system. Most gear made for the caravan, motorhome and RV world is made for 12v and the 24v versions are typically more expensive. I am using a Redarc BCDC1240 charger which happily takes power from the 24v truck system or solar panels and charges the 12 house system using a 3 stage protocol. This removes a lot of the hassles and if you search around you can get good prices. Given this is a key unit in my system I may carry a spare.
Lighting is easy nowadays in that most LED lights are made to run from up to 32 volts and the cost has dropped greatly. I can't understand why people still use flouro, halogen etc. They must not appreciate the current draw.
Having a totally self contained system is wise. Whilst grey water tanks are not yet mandated they are required for National Parks and on a recent trip to NZ we found that all motorhomes there have to be self contained. My guess is it will happen here soon. I am putting in a shower as the 'chief architectural approving officer' stipulated it as non negotiable !! This means that our water capacities will be up on yours. We have always had multiple tanks in our vehicles so that there is redundancy and I try and make sure all water is drinkable unless desparate. We have a good filter system which will be plumbed so that I can move water between tanks and choose which to use. In the past we have found when travelling in remote areas where the water quality was poor our approach worked well. This includes o/s in regions where the water borne risks were known to be high.
Will finish now and pardon the long winded comments but I have spent a lot of time thinking and researching before starting the build and even now I am changing things as I actually construct things.
Happy to exchange ideas and knowledge if it is of use. Please do not speed ahead and get yours finished quickly as a few of us will be very jealous.
The 20mm spacers between the chassis and subframe will be replaced with rubber when finished.
The step on the chassis has two distinct angles.
The mounts will be fitted with 2 100mm springs to allow some flex of the chassis.
The rear mounts will be of the pivoting type to allow access to the starter motor, transmission, and underbody tanks by jacking up the front of the camper.