I have a '71 TT and have a single tank. It may be that some of the newer,smaller trailers are provisioned the same. I do have valves that separate the shower and sink water from the black tank so that the waste water from those can be dumped through a garden hose outlet into a tote.
Pouring water down the shower drain and having the grey water drain open should show how the shower is plumbed. I don't think there are many trailers that are plumbed like your present trailer. There is little need to do so.