here's the piccies of my truck for some ideas

Foley wheelchair accessible vehicles
 

Have a chat to Foley's - when I was there a few years back they were building a 6x6 Landy for a wheelchair user - complete with Dormobile interior etc - they can build up whatever you want (not cheap, but good quality!) - they are just north of London if that is of any help - or the website can be found through Google....
Tony

Have you looked on the SBMCC website? Lots of people who have been there and done that as far as building their own camper to a greater or lesser extent, although I don't visit enough to know if any were in a chair.

Haynes do two books on converting one yourself but have rubbish reviews. Travel Vans by Tom Speed is really good, even to give you an idea of what detail is involved, or how something you've bought should ideally be doing. Well worth it!

Travelvans

Willinghams of Hull built the box we're living in at the mo, and although a few details could be better, and its certainly not light, it does the job well.

Rob Gray :: graynomad :: wothahellizat has some cool ideas for a big truck, such as a months worth of home brew under the floor :) as well as roof join construction for instance which is a poor detail on our mog.

If the rear seats folded, or you only had two seats with a gap between, then with you sleeping inside couldn't you sleep half in the front and half in the back? Then the vehicle remains secure, and noone outside would know anyone was asleep inside? May be useful in towns, ferry ports etc?

In my own opinion, if you start from a 4*4 with outside access for most of your stuff, I'd think you would struggle to reach it, to make the most of the space you have? Especially in the uneven reality of where you're going to take it, the distance floor to truck bed could be quite variable. You could be better starting with a bare chassis behind the crewcab so you can build access as low as poss. Starting with a Hilux with pickup bed has quite high sides. An ex Army LWB Landy used to at least (still does?) have removable sides above wheelarch height, allowing you lower access? A panel van built without chair access would at least make it easier to get in (cos the doors are lower), and then clamber up, above and therefore access, anything there? (I apologise if that would be too undignified. My imagination says that would all be part of the fun, assuming its possible for the people you would take)

Or, would it be feasible to use pulleys and/or a winch to raise you in your chair so you could more easily access a more standard (and cheaper) truck? We had a simple 4to1 rope pulley system from a ship chandlery to put our 100kg spare tyre onto its perch by hand which wasn't too hard and could lock in position, but only lifted from one point. A two point lift at two positions on each side could be there ready for use all the time? A sort of 6' abseil :)

Or unfolding sides so your stuff comes down to you :)

Be careful about adding to much weight though.

Good luck :D