sink waste

Make a sketch of the pipe layout under your van and then compare with other same-make vans. 

Which outlet does your bathroom sink drain from? I joined the bathroom sink & shower drains together on my last van. Not sure how my Coachman's drains are connected,  but think it's  like your Elddis, though we don't have the same problem 

Plug the end and pour Coca Cola down the drain and let it clear everything.  My van has the shower on one drain and two sinks on the other.  I would prefer the two bathroom wastes on one with kitchen separate.

There's two interchangeable threads running on this but not every answer on each.

There are two thread running on same subject from OP

I think normally the shower and sinks are plumbed in separately to prevent this. However, if you join the two together as soon as they exit the van, it is possible to get back pressure. This is not normally a problem for us with the wastemaster, as long as the single feed (from two) into it is not obstructed. However, we get it a lot on service pitches, where there can be 3 or 4 metres of flexible waste tube running along the floor to the drain, resulting in backing up.

Had this problem only once on our Explore, easily solved, the problem was caused by the "y" pipe into the waste master touching the bottom of the container, forcing the waste water back up and into the shower tray. I make sure the pipe is sufficiently raised hence no problems since.

will look under the van tomorrow but think you are right and the sink pipes have been connected wrong

there are two threads because I edited the first from sink waste to sink waste problem but it made two threads

Both sinks should be connected to one of the outlet pipes and the shower tray to the other. Because the sinks are at a higher level than the shower tray if either are connected to the same pipework as the shower tray you will get back flow into the shower tray. I am fairly certain that this will prove to be the source of your  problem.

The shower needs to have its own outlet, 

2; some caravans have a long route just to have your waste at the side, ours use to go past the axel then double back with a few bends, I changed it to discharge at the back, it now has a good flow with no problms