Unlike the Prius it only has an electric drive train with a gas generator.
And it's a hatchback with the seats folded you might just be able to sleep.
The volt is a sedan, but only 2 seats in the back split by a center console. I don't think that'd be very comfortable without an air mattress.
Here's an image of the new 2016/2017 volt with its seats folded down: http://www.tampabay.com/resources/images/dti/rendered/2016/0...
My guess is the OP is a climber or similar and has pretty specific van needs.
With own car one pays insurance once, while renting the insurance gets payed every single time it gets rented.
Not going full coverage is an open game as they will charge for any little new scratch the car might get, no matter how tiny.
Meanwhile, I have been in countries where the only thing that matters is that the car is returned in one piece still.
EDIT: The car hire companies make a massive profit on the 'super cover' optional insurance.
If you are worried about the provenance of a financial services company, there are usually national regulators who maintain registers of companies that have to abide by certain standards. You should check that an insurer is on this register before taking out a policy, as well as checking to see if there are a lot of complaints about them on the internet.
If you don't mind doing the work yourself, and don't mind a range lower than 500km a day, you can get something like this for less than 30k Euros. For example:
http://hackaday.com/2017/05/17/solar-powered-camper-is-a-mag...