[PARAPTOR]

I assume you have used your rig more than once? Did this problem just start or was it like this from day one? When you say Dirty if you are referring to the black tank dirty you could have something lodged in the valve/seal like toilet paper.

If this is the case you may want to consider doing a few flushes of that black tank before taking it to the dealer and waiting to get it back for a few weeks/months. As far as warranty work not sure a clogged valve would get approved in comparison to a faulty one. You may be liable for the labor to clean it out