Here are some thoughts on your ideas.

35 foot box will require a tamdem, which you show. The cab on the chassis will be a minimum of 10 feet bumper to back of cab, possibly longer. There will restrictions in some states on what roads you can use with a 45 plus foot unit. Mostly restricted to interstates and major two lane highways. Might be problems on secondary roads.

Interior lighting. Make sure the light is over the front of your body, not behind your head. Behind your head casts shadows on things like reaading materials. I have seen this mistake in at least one manufacturer's layout.

Extended cabover, slide in campers have them. Ask manufacturers. The only problems I can see would be possible blocking of engine compartment for major service and front axle weight.

Bathroom layout. How are you going to be able to use the sink with it stuck so far into the corner? The wall in front of the toilet could be a problem. If you would suffer a knee injury, like a torn ACL, you need to be able to sit on the toilet with your leg fully extended. You cannot bend your knee for a few months. Solution is to move door to center hall. If necessary the door can be left open to extend leg and the sink wall can be extended so the sink is more accessable along that wall.

200 gal fresh water. Will you have large enough waste water tanks to match?

This is going to take a very large chassis with a lot of axle rating. Will even a 12,000 lb front axle rating be high enough. For example, I have been told that Kenworth T-2000 chassis with a 12,000 lb front axle come close to be overloaded on the front axle with some motorhome designs.