Thoughts on water tank/gas bottle & heater.
Why not mount your water tank under the wheel arch on the outside of the vehicle?
Put your gas bottle on the other side to give some counter balance or mount two smaller tanks, one on each side.
I know you mentioned earlier about putting you gas bottle under the arches, but decided against it on the grounds of difficulty of access and safety. If you made it reasonably easy to remove for off roading, then the safety issues would be lessened.
I would probably use two or three ratchet straps to lift the bottle off the ground and thus secure it - though an added metal strap would be worth considering to lock it into place or a rack which the bottle lies on and the straps used to raise it up. Then you could simply fit the regulator while the bottle was at ground level and ratchet it up under the wheel arch from inside - if you pass the straps through the top of the arch that is, the same could apply to your water tank or tanks. Or if your water goes under the arches, then the gas bottle could go at the bulkhead.
I have an eberspacher heater in my 110 and I have mounted it on the access plate where the centre seat used to be, I intend to build a cubby box around it with a false floor which will cover the heater.
As for turning the floor "inside out," I had been thinking along the lines of making two cuts from floor to sidewall then (and this is the bit that would be difficult) re-bending the original box section so that it would "invert" into a flat floor section and fill in the open spaces at the ends with a small plate. I was then gonna mount my generator on one side and a porta potti in the other side. But, my generator has been living in the back of the van for a few months as I'd been using it and I kept noticing the smell off the petrol and I don't want to be sleeping beside that all night, the porta potti I could live with as it would only be used in emergencoes and cleaned immediately after. So the generator idea is out the window and will go back into a little trailer I used to use for camping gear (genny, microwave, toaster, kettle, big gas bottle, picnic bench set etc.)
Since I abandoned the genny idea, I have come up with the plastic tank storage box idea, they would be useful for just about anything to be kept in out of the main living space in the back. Cutting the holes in the top of the arches will be easy enough and the tank will simply fall into place, sealed round the edges with silicon sealant and a simple hinged lid for a cover, or no cover at all as I plan to make bench seats which will double as beds similar to the way they do it in caravans, then I can simply lift the base of the bench for acces to the contents of the tanks.
P.S. I don't know about water heaters, but the Eberspacher night heaters make very little noise. When they start up, you will hear it, but once the temperature has risen it goes into a sort of tick-over mode and makes very little sound. Certainly, nothing to keep you awake, or even wake you up once you become accostomed to the sound.
