Make a big bed, storage underneath, insulation, ventilation and keep all ya water wet stuff and cooking outside if possible.
We lived in an old Roma Supreme for years, a "dry van" That's the way to go where ever possible. If you can't do that, run a dehumidifier in your van from time to time.
We've got a 1972 T2 Dormobile, had it for more than 30 years, it's very well designed. I'm gonna let it go soon and buy summit I can have a permanent large bed. Also I'd like something I can stand up in, make a cuppa tea and the odd one pot meal. A fridge is a nice luxury, may be a small wood s