Can you see the shower drain line from bottom of shower down to top of tank?
Or is it hidden behind one of them false walls/panels in basement compartment?
Bathroom sink and shower drains in your rig probably tie together and use a common 'P' trap.
You should also have 'air admittance valves' under kitchen sink and bathroom sink AFTER the 'P' traps to keep them from sucking dry when you drain grey waste tank(s).
IF they don't seal correctly stink from grey tank will get inside RV.
GREY stinks......sometimes worst than black.