The vent was under the tub, which was raised to allow for the plumbing.
There was a vent and a t-paper holder. The paper holder was a bad location, the paper would get wet when getting out of the tub. The vent put too much heat into the bathroom, and none to the rear of the camper. The t-paper was moved to a different location.
I rerouted under the tub to the hall wall, and covered the tub side with material that was left over from the tub surround that I had added (the unit did not come with a tub surround).
If your shower base is raised enough to allow space for the vent duct to go under it (maybe around the back side), you may be able to fish is thru and up into the shelf area of the rear bunk. You may have to take the rear shelf/panel
out to gain access.
Our current camper has a vent under the tub (small dia. duct, so a smaller amount of heat vs. the other vents), and the duct runs thru the bottom of the vanity cabinet, thru the wall to the lower front bunk area.
Good luck and let us know what you come up with.