I’ve been involved in a few “camper” conversions over the years, the biggest being my 6 ton Dennis fire engine which I lived in for 2 years in Normandie. I also did a 4 ton 4 x 4 Dodge fire engine which included a hidden tail-lift “garage” for a motorbike in the back and still looked as though it was a fire engine.
On that point, one thing worth considering is the restrictions on parking a “camper” just anywhere, to which end it may be worth considering a “blind” van: a camper that just looks like a regular van from the outside; nobody bothers where you park a van but there are often restrictions/local regulations against overnight parking of campers.
My current vehicle is a 3.5 tonne Peugeot Boxer L2, H2, which gives you 3.25m long in the back with full standing headroom and sufficient width to put a bed across the width but not too ridiculously long for parking. I’m thinking that I may get away with a smoked glass window on the sliding door that can be completely blacked out with a dark curtain but, as I said, if you make it too obvious that you’re kipping in it and you could get moved on from certain spots.
Another recommendation would be that you make/instal things in easily removable units so they clip/strap/bolt in place; it will give you more versatility. Also, if it’s just for camping holidays, you shouldn’t need anything too serious for cooking and washing up, etc as you’ll probably find yourselves dining out for most serious meals.
Well, I would; I’m not a big fan of cooking.
Another thorny problem is the “thunderbox” - apart from a curtain around a Porta potti, I have no great solutions to that other than keep a box of matches handy.