I knew it was a Studor; could have saved you the time
The shower/sink distance to the tank is so short venting those runs does not matter. It's when you come to wanting to drain the gray tank without it gurgling or maybe sucking the water from the drain traps it becomes better to have a vent. Studors work, but then I've never been one to worry about cutting holes in roofs. I do that on houses a lot.
After looking things over this AM I decided to use one of those Camco sink drains on the shower too. It makes the task easier and it runs well. 3/4" ID hose with a PVC adpater and a Uniseal grommet makes an easy quick connection to the tank.
The HepvO will go into the cabin so I don't have to remember to drain the sink trap there, or remember to add antifreeze in winter.
I think I'll tackle the Fantastic fan this afternoon.