I'd imagine the OP is familiar with wet wipes and alternative ways of bathing, having spent three years on a motorcycle. I think this has taken a turn towards camping trips in warm climates rather than eight months in the stans and Russia. You can't really go eight months without part of it being mosquito season and part of it being cold and wet. In my (very) limited experience with sprinters, if it was my van, I'd go for a large wet bath taking up the whole rear since the T1N LWB is a pretty big space. That way the rear doors can be used to service the Nature's Head or cassette toilette or whatever you go for without a bunch of bending and finaggeling though the living area, and makes it easy to clean and ventilate the shower area. When the weather is nice or on the beach you can take a shower with the doors open if so inclined. That way you're also not tempted to use the rear as a garage full of junk, or be too tempted to hang something heavy behind the rear axle - the water tank can still be mounted on or in front of the axle. Since it's a large space, I'd also imagine that with some clever fold-down-or-up thinking you could get a sink that you could use both outside and in, for washing dishes or clothes or whatever, removing the need for an extra sink in the kitchen area.