Hi all. Had this seemingly common problem of mid kitchen with very slow sink drainage. After checking the pipes for blockage (clear), I noticed the drain pipe being attached to the underfloor had no drop, (the van sits very level), so gravity effect was all but nil. I bought a water pump (4l/min) and installed it inline behind the fridge via the fridge vents. Wired it to 12v supply, operated by push button near cutlery drawer. Works well, but it's fair to say a larger capacity pump would be better. Hope this helps folks.