In post #1 of this thread
, you can see the arrangement of the vent line behind the tub surround. In post #21 you can see how the new FRP partition fits up against the remaining half of the tub, and if you look closely you can just barely see the new white PVC vent line protruding through the horizontal part of the surround. You're not interested in making the shower area smaller, but maybe this will give you some ideas. I wanted more counter space in the kitchen and was willing to sacrifice the shower elbow room. This winter I am going to remove the entire plastic "sink thingy" and redo the shower.
You can see in the photo that one leg of the black vent line goes into the floor to vent the black holding tank. The other leg circles back along the wall to vent the sink drain line. This sink vent is overkill (in my opinion) and the sink could easily work without a vent at all (oh, heresy), if you don't have a gray tank--there is another vent for the kitchen sink and it will prevent any flow from the kitchen or shower from sucking the water out of the bathroom sink P-trap.
You can replace the shower P-trap with a Hepvo (that's on my list, too), but you still need a couple inches of elevation between the bottom of the shower pan and the top of the gray tank (if you're installing a gray tank). Otherwise, you can remove the shower pan/tub platform and put the shower pan on the floor.