I have a 2014 Keystone Alpine 3600RS
After I drain my black tank, have sewer smell from the shower - 99% sure that the trap under the shower isn't a P Trap!!!!! I believe that the problem is the HepvO waterless P-trap malfunction - look it up and see what you got - it's a tube that has a rubber hose inside that opens for water to go out - - if it doesn't close correctly, sewer gas will come into the living space.
check this video -