I admit to being at a loss why a car rather than a van, tent or trailer tent, but....
If you must sleep in a car, a big older diesel estate will get you great mpg, very good range and with the seats down you get a space which is longer than you and wide enough for 2. Overall cost to run (purchase, insurance etc) prob less so long as you choose the right car and either service ut yourself or use a good local independent garage.
What do the wife and daughter think about this idea? A fancy mattress in the back of a Tesla is all very well, but unlike a hotel or campsite, it lacks showers and a toilet. (Don't underestimate the value of a toilet to most women......)
We don’t camp often, usually less than 4 days a year, I want to increase this. However, my wife and daughter prefer me booking a hotel, and I find this expensive. I hope to increase our camping to about 2-3 times a year for 1-3 nights each time, probably 4-6 nights in a year, don’t think they will want to stay out for longerAlso..... you need to travel a lot to recoup the cost when comparing this kit out with a night in lodgings, or a campsite pitch with a decent tent.
I am likely to stay by myself most of the time, but cannot say how often - maybe only twice a year hunting or fishing, maybe 52 nights if I have a contract for staying in another town. Also, I will be using the vehicle as a car like shopping trips or go to the gym, so I would think a van will be more expensive to run