First off, I don't like carpeting in any RV. Carpeting is a nice way to cut assembly cost. Much preferred is hard surface flooring. You can add throw rugs if needed.
Secondly, black mold is deadly. Time to remove ALL the carpeting with a NOSH approved face mask or supplied air resperator, you don't want to breath in any mold spores.
If it's happening along the wall / floor juncture, you have water intrusion in that wall and most likely soaked insulation and rotting framework and that is a major, major job to remediate.
I have to ask, have you ever checked your seals and caulk for failure areas? I bet not. You must at least bi-yearly, check all the seals and caulk seams for failure and reseal and re-caulk as necessary or...you wind up with what you now have.
My personal view is, the dealer fixed the issue but follow up seam and caulk maintenence wasn't done by the owner.
I've seen brand new, on the lot units with water issues pending. Just pulling a unit from the factory to a dealer can compromise the seals.
Maintenance is an important (but overlooked) aspect of RV ownership. Now, you get to pay the price for your lack of care.
Interestingly, you can purchase Eternabond and Dicor at any home improvement store, Sikaflex too, but you have to use it to do any good.
If it was my unit, I'd patch it up and trade it in. Because you are looking at not only an expensive job (remediation), but a long time duration as well.
No way to candy coat the obvious.
2015 Backpack SS1500
1997 Ford 7.3 OBS 4x4 CC LB