Hey I live in New Zealand but hold a South African Passport still. I will be travelling around Europe with three of my mates in a campervan from August and was wondering whether you still need to show proof of accommodation for the Schengen visa even if I will be living out of a campervan for the entire trip?
This requirement is indeed often problematic—if you want to travel around and do not know the exact schedule yet, or if you want to stay at your friends half of the trip. The solution I've been using extensively is to book a hotel for the whole period on any booking site with free cancellation (they often do not charge your credit card until a couple of days before your arrival), get the visa with the confirmation you receive from the site, and then cancel the booking.
Another argument an embassy may consider is the amount of money you can prove you have. Usually they would require you to have some sum per day of your stay. If you bring them the bank statement showing that your minimum monthly balance over the last months is above the requirement, they may accept that as well. You can find the official information per country here.
However the final decision is very much embassy-dependent—both on in which country you apply and at which country's embassy. Therefore it may be advisable to go the first way.
Also the return ticket is always a strong argument, though some embassies actually advise not to buy air tickets before obtaining the visa.