I think something basic like a lined and insulated van with a raised floor and two slide out drawer units underneath would be great.
One drawer to hold the kitchen equipment, fresh and waste water etc and a shower head attached to a hose on a 12v pump for cleaning duties. The other drawer partitioned out for sleeping gear/clothes storage (could even take a small porta-potti).
A couple of airbeds or inflatable roll mats would do for two people sleeping on top of the raised floor and you could even add internal attachment points to sling a hammock inside for a third berth if required (driver and double passenger seat up front?).
Folding table and chairs for outdoor use that are strapped to a wall inside when not in use. Something like a foxwing awning attached to sit/cook/tinker with bike under.
Add an electrical socket or two for charging duties, some lighting and a cool box. Diesel heater would be good for colder months. Bungee netting across ceiling to act as loft storage/hang wet gear to dry etc and you would need some means of securing bikes and other kit for during transit.
If you offered something like that it would need to be at the right price between full on campervan hire and commercial "white van" hire rates. I would take one a few times a year. In fact I might have just talked myself into swapping the car for a van of my own!