Blocked kitchenette sink pipe in caravan

Hi all

Looking for some advice please!

We have a blocked pipe from the kitchenette sink. Blockage is near the exit port as the water just goes straight back up into the vanity sink and bringing stinky mud with it (sounds and smells like a mud blockage). We use a basin for the dishes and just empty dishwater into an external drain so it hasn't been an issue as we don't normally drain water down the kitchenette sink - there's a small crack in the vanity sink and it drains (slowly) from there into a small bowl that we keep under the crack for when we forget! But would be good to just get it all sorted as we want to leave the caravan somewhere for the season next year and family members will probably also be using it.

Anyone experienced this before? I don't want to go throwing chemicals down the sink as afraid of damaging the pipe, but is there something (preferably a natural solution as not a fan of chemicals anyway) that will break it up?

Caravan is an Elddis.

Thanks in advance
ccl