Hi Alec, Good luck with the van, I found fitting the bonded glass a very rewarding task! As mentioned above, get cracking with the insulation. I've used a combination of various types depending on location.
You mentioned a high level bed, does that mean you're going for a permanently fitted type? I have in my van, It's a permanent king size. Getting old now so comfort is top of the list! It's about two feet off the floor, so have useful space below. My water tank sits between the back wheel arches (under the bed). I intended having the two leisure batteries at the back too, but found the van was getting out of balance, so have moved them forward to just behind the cab area.
After fitting the floor (with insulation under the boards) I built the sturdy bulkhead between the bathroom/kitchen area and the bedroom. This gave me a datum to build from for the rest of it. As I'm building to a budget, it's not a shiny interior with fancy fittings, rather just a good user.
Anything I would have done differently? Been much more on top of the wiring as I went. I guess that this is down to more accurate planning!