As previously posted, loads of info on expedition portal about fitting out campers/vans for long term use.
No big deal living in a van if you have facilities at work I've done it for the last 3 years, as you say, far better sleeping in your own mattress and a load cheaper.
I used a Disco 1 before getting my RB, compact and bijou but standard bushcraft sleeping gear keeps you plenty warm enough and comfortable, a comfortable bed makes a huge difference to the experience, add a second battery to run your toys, laptop (get a tv tuner if you want one for the laptop) etc. LED lighting uses bugger all power, go for a roof light rather than side windows for better stealth, blackout curtains for the front and rear windows.
I know people recommend kingspan etc but I used cheap sleeping pads contact spray glued to the side panels, very flexible, easy to cut and thinner, I was car camping in -12 degrees without heating.