I don't have any direct experience with that problem.
I'd go to another dealer and show them the file I've attached showing the plumbing system in your unit.
I highlighted the sink and shower plumbing in yellow and circled the shower p-trap in red.
I'd check the p-trap first. That can be done from underneath the van I'd expect.
If the van has been in freezing conditions and rv antifreeze was not poured into that p-trap then it can crack if the water in it freezes.