Take a look at
Interior pictures of my van for sale » Sprinter Westfalia Westfalia have been making campervans since the 1950s (I think) on the VW Kombis originally, but now do the Sprinters.
As others have said - water and batteries in the middle of the van to keep weight distribution. If you want to make really sure that weight is correctly distributed there are plenty of formulae available on the 'net that you can enter into a spread sheet and work out how much weight goes onto the front axle.
https://til.scania.com/groups/bwd/documents/.../bwm_0000461_01.pdf
Having a wet area with shower curtains is a recipe for problems with either rust or flooring getting affected by water - stick with a fibreglass shower cubicle for longevity and privacy, they double as a toilet as well. Many of the shower cubicles come with pre moulded sinks/toilets/mirrors/lights. Put a vent in the van roof and those human smells wont be a problem either.
Side facing seats are these legal and how will you mount them, you need substantial mounting points in the floors for front facing seats - side facing would be even harder to make comply - they are pretty useless as well if you do have an accident as seat belts work on forward movement of the passenger and not sideways movement. I think I'd put two Rock N Roll bench seats facing forward, with inertia reel belts - only two seaters as you need space to walk down the living area. Have a look at some photos of late 1980s James Cooks on the Merc T1 Chassis.
Rock N Roll seats/beds are the way to go if you want to maximise space. Have a look at some of the older James Cooks with single seat/bed RnRs.
You can put a shelf above the driving cabin and convert that to a bed with a simple slide out.
Gas - put a gas tank under the vehicle, fill up with Autogas, far easier than bottles even with Autogas fillers.
Cabinetry - door cutouts need to be rounded and the doors (when closed) fit inside the cutout, this strengthens the cabinetry and will make it last 30 plus years, plus you wont have doors falling off when you drive down a cobbled road.
Wiring, isolate your 12V and 240V circuits, 240V circuits should be in conduit.
Flooring - one layer of 3mm Masonite, one layer of 19mm fibreboard, one layer of 3 mm Masonite, then vinyl floor on top. Use the Masonite to layout your floor, where gas, water and power lines will cross the floor.
Use T bolts to hang anything on the underside.
Two vents - one at lowest point, one at highest point - always open.
Open fire inside a camper van is not a good idea in my opinion, if you need heat buy a combo water and hot air heater that works off diesel from the vehicle tank - cheap running costs. While I'm at it, buy a Webasto diesel cooktop and then you don't have to worry about gas at all if you have a good compressor fridge.
My two bobs worth as well.