Being an uncool SE owner, crocs all the way. Left under the van at night, shower in them if facilities a bit grubby, great on wet grass ( no soggy shoes), stoney beaches etc. Very handy, light and can be scrubbed. I just cannot use flip-flops.
Crocs away for me! Great for wet grass and showers...I can't do mules so the shoe types are just perfect. I also take the free spa slippers. And for Christmas I got water shoes but I don't think I'll use them in the van, just on sunny beach holidays....they're more to protect my feet from shells and stingrays...I'm not planning many trips with my van outside the UK. They're good though!
+1 for crocs (or Lidl cheap versions) excellent for the wet grass and dodgy showers. They live under the sliding door or in a bag on the doorstep at night. Nice thick woolly bed socks for inside the van. Teva's for him, Keen's for me if wading is required