I envision it with the fresh water tank under the fridge, batteries in on the floor left of the water tank, dual 20lb propane tanks under the counter in the front. A section of counter top could bridge between the counter and the fridge.
Other than switching the propane and water tank that is generally what I'm thinking right now. I was potentially thinking with the propane being under the fridge I could have a sealed off access hatch from the inside but still have external vents (need to verify the legality on that). It would allow access to the compartment when something other than propane is in the one spot along with the ability to deal with swapping propane tanks/turning them on/off, etc. and being shielded from the weather.
Something else to think about is a simple 7 gallon portable water container for the gray water.
That is where I'm leaning now. I'd still likely put a Y valve or something in the line to either go to tank or to an external drain. I was thinking a tube out of the front of the camper going down between the flatbed and the truck would be inconspicuous (but a wet spot under the truck might catch more attention...)
I would put some though into the full height door before you commit to it. I think you would sacrifice the structural integrity too much.
I've been stewing over the door thing a lot and still don't have a great answer. The main issue I run into is since I'm doing overlapping hard sides I can't just do a typical 4' door and still be able to access it when the camper is down. One thought I did have is if I did a split vertically hinged door like sonke's rig (if you aren't familiar the picture is below). The bottom door would still be able to open with the top down so you could slip through the ~20" gap there but its definitely a compromise.