So I am very firmly in the camp of people who prefer to sleep in the vehicle (or in an integrated camper top) rather than on a separate RTT. Weight high up, insulation warmth and general nimbleness are driving factors for this in my mind. I do this very effectively in my G-Wagon today.
When I saw the Grenadier in person at SF, these were my observations. The length from the back of the front seats to the back door is about 64-68", which isnt much..but because the Recaros are manually adjusted, you can quickly drop the back rest of the front seats and have a proper 78" sleeping length. So about 6' tall people can easily sleep inside.
I was told that the rear seat design is still not finalized as to how it will fold and/ or tumble. So it could fold flat or tumble in parts like a 4Runner. That may or may not change the sleeping possibilities. I intend to remove the rear seats and install a flat plate (like Goose Gear plate system) to create a blank canvas to build on so its not an issue to me. By my visual estimate, the aux battery, fuses and other paraphernalia under the seat was tucked well under the cargo load floor line. So I dont think a flat surface from cargo door to the back of passenger seats is an issue
The challenge is in the vertical height. I love to sit inside my rig during bad weather or in the evenings reading a book after everyone has retired at camp. My G-Wagon has a vertical space of ~48". That allows for a 10" storage space under the sleeping area and the ability to sit up comfortable for my height. This is even better in LR3 and LR4 with the kink in the cargo area roof line. The Grenadier has only 42". So if you put in a 10" drawer/ cabinet under the sleeping platform, you are left with ~32" of torso space to sit up. Thats the only challenge for me on the Grenadier.