In summer i'm a scubadiver and in our club there's a guy who makes almost 200 dives/year and basically lives at the coast.....
he has bought a large van, put a large bed right behind the 2 seats, behind the bed, a living area with a bench, table, tv, stove, cooking table, sink, etc...
underneath he attached a water container that can be heated; when he comes out of the water after a dive he can attach a garden hose to that container and take a hot shower :)
+ he has a sunscreen that can be extended from the side of the van so they can chill outside :)
quite an epic van, much larger and comfier than a VW van, thats gonna go at least 10k to begin with, while he spent 12k on everything....
also make sure the van can be vented properly, sleeping inside a cold vehicle will cause lots of condense and make everything humid/moldy ....
^^the guy has 2 small side windows and installed a window in the roof ....