Some of my suggestions:
Sat nav - no arguing over "You should have turned left back there!" or "you didn't tell me in time!" with the navigator and her paper maps!
It's also nice to see how far you've still got to go, and what time you should be arriving.
Some way to keep warm - a firepit for outside in the evenings - it only needs to be a simple bowl type, even a cheap round BBQ, which can fit on the roof rack upside down (watch the legs though

) and you can buy charcoal almost everywhere so you don't need to haul logs around.
And, of course, a Diesel powered cab heater for warming the vehicle up when getting dressed/undressed - that's my favourite luxury
Nothing worse than being cold and miserable when it's supposed to be a holiday!
Proper pillows for sleeping and for the navigator to rest her weary head against the door pillar! If you have proper down filled pillows and a stuff sack they don't take up much room at all.
Fridge - not strictly necessary, but a small compressor fridge for milk, cheese, meat and beer is useful. Yes, you can and probably should buy local, but it saves having to find shops every day - just one less task.
Then of course an aux battery, and if you are not driving every day, a solar panel to keep it charged, unless you get a 12V/gas fridge, though you then have to remember to turn the gas on and light it when you stop.
Having places for everything helps as well - it's not good piling everything in the back only to have to pull it all out and stuff it under the 'wagon in the rain so you can set the bed up or even get to the cooker and food! If it doesn't fit, see if something can be left behind! Things like collapsible bowl, bucket take up less space, and I know Alex and Esther only take the clothes which fit in their Dormobile wardrobe. If it doesn't fit, it doesn't go
Cheers,
Robin.