Re: Bus Plan Picture
Consider that the shower drain can't really be much higher than floor level, so without pumping your gray water, your shower location will likely dictate where your graywater tank will be located. If that's the case, the graywater from your sink(s) has to get there by some path via gravity (1" of drop for every foot of horizontal travel, IIRC), and if you have to cross frame members or axles, it can get interesting. Freeze-proofing any exterior pipes might also be a consideration.
And I love the idea of a rear deck as much as anyone...but there are some challenges to making it a useful space. First, the access door is 4' tall. The deck in the sketches is 2' deep, which isn't really a lot of working space to do much, unless the grill/table is cantilevered out somehow. Also keep in mind the door will swing outward to open, so it will eat some space. IMO a rear deck is great for storage & maybe an extra access, but as space for a cooking/chilling spot, you might be better served with a 10'x10' pop-up shelter & camp chairs.
Just my opinion on all counts. In any case, please keep the rear emergency exit accessible & operational.