There is a vent for the shower on ours, beyond the trap under the shower floor, but because the subfloor is obviously too shallow for a riser the vent itself is behind the adjoining galley area cabinetry. It took me a few years to run across it. That said, if a vent fails or sticks, when you dump your grey tank the suction that action creates can empty the associated fixture trap. Then residual tank odor can back up into the coach via that drain as it has occasionally with our shower. It took me awhile to zero in on that as the source, but simply adding a cup or two of water in the drain refilled the trap and stopped the odor path into the coach.
Some have found their roof vents improperly working, such that traveling creates a vacuum inside the coach and drawing the smell in that way. As others have noted, when the washer is not regularly used and it’s trap is empty, that’s a common source of tank odor; and if the gray water roof vent is somehow compromised and a coach window or roof fan/vent happens to be open while driving... well, there you are. Several cups of water in the washer drum and one spin cycle should refill a dry trap, as David suggests. That’s how we add winterizing antifreeze to the washer trap.
Others here with that past experience will have to provide their tank roof vent fixes.
Joel