Seals Maintenance

Fixed your photos.
I can’t think of anything mechanical that would leak from behind the passenger side wheel except the Alde convectors.  Check under the bed/bench inside.

@Leb, it is not clear if you looked under the bench seat as suggested by @Sharon_is_SAM. Take a look at the below discussion. That is the area I believe she is referring to, because it is a common source of leaks, usually from inadequate or deteriorated caulking in the area of the rear lights (behind the plastic shroud/cover.)  If you had a lot of rain, it may just be residual water rolling down (or built up inside) the back of the trailer behind the plastic shrouds/covers at the rear of the trailer, which isn't necessarily a problem.  BUT, if you see signs of water damage inside the trailer on the wood at the rear sloped area under the bench seat, then that is a concern that needs to be addressed with proper caulking.

https://tab-rv.vanillacommunity.com/discussion/14526/floor-repair-due-to-rear-leak-in-2018-320s#latest

https://tab-rv.vanillacommunity.com/discussion/8446/leak

@Sharon_is_SAM, I think this is the discussion you are referring to.  Thanks for reminding me about it.  @Leb, you will want to review it as well, because it was what I was referring to as another possibility.

https://tab-rv.vanillacommunity.com/discussion/7120/2018-t-b320s-water-dripping-from-under-rear-trim-and-undercarriage-water-barrier#latest

@Leb, also take a look at this:

https://tab-rv.vanillacommunity.com/discussion/6393/

As @Horigan has suggested, always use a urethane or polyurethane based caulk/sealant and NOT silicone on the exterior of the camper.  nüCamp uses Geocel 2300 MHRV, a marine type urethane or polyurethane based caulk/sealant, but I am not sure what they use under the wheel wells, but maybe it is the same, only in a black color.   A similar product, which may be more readily available through most RV stores (e.g., Camping World) is Geocel ProFlex RV Sealant.  I believe you can get it in different colors (e.g., white, black, clear.  My notes indicate that the "clear" version product number is GC28800, but double-check that if you are looking for a clear sealant.

If you search this forum (using the search box at the top of each page) for "sealant,"  "caulking," "seal maintenance," or similar words, you will likely likely find plenty of discussions on this topic. 

You would think that if NuCamp went to the effort to create the seal inspection instructions on how to reseal the trailer, it would include the recommended sealant(s).  It was surprising how much searching was required to get to these recommendations.  I went down this rabbit hole last week.