Yep, the drain tube is blocked with gunge from soap and hair, just like the sink at home. You'll probably have to remove the tip up sink to get to the drain hole.
I think I had to flex the outer bit carefully and pull the sink out gently. It's twenty years since I had to do it and I only had to do it once, luckily my more recent vans have had more conventional plumbing.
Thankyou so much my hubby has managed to get the sink off and it is chocka with soap and scum and all sorts good grief . We have bought it pre loved apparently not very loved behind the exterior, we are on with it now. Once again thankyou so much