You don’t say how old the van is but if bought from a dealer and you can’t solve the problem you could try calling them for advice.
It’s a 2018 model, the other 12v items work such as the radio and blow heatersAre other 12v items working? Can’t think that the external pump and the loo flush have any commonality except being 12v. Not knowing your caravanning experience it is difficult to know where to start. There may be an overall fuse on the +ve side between the battery and the electrical unit, so if no 12v worth following the +ve lead to find and check it! Also worth checking the security of the battery terminal clamps. Please keep the forum updated!