Very happy to report back that I found an easy solution that I hope works. I found some flexible pvc at Home Depot - it was labeled as spa hose or something and you could buy it by the foot. It came in 1", 1.25", and 1.5" inner diameter. I went with 1.25" to make sure it would fit in between the walls, which it did perfectly. 1.5" would have been too fat.
I patched up the galley roof vent opening because I won't be using that one. Instead, I'm using the other vent opening that was back by the bathroom. This vent I think was originally hidden by the shower and actually ran outside of the wall, so there was a hole cut in the fiberglass endcap where it went through. But I wanted to run the pipe inside the wall since I'm downsizing the bathroom and that area will be exposed by the bed.
Anyways, I was able to get the pipe up through the vent and then run it down between the skins along one of the ribs. I just popped a few rivets so I could pull the endcap back enough to fit it in. It was actually much easier than I expected, luckily. The pipe bent along the curve perfectly and is now inside the wall. You'll see in the pic below that I need to replace the vent gasket to seal that area up there. But once that's done hopefully it'll be all good. I got a step-up fitting to take the 1.25" vent pipe to 1.5" so I can connect it to the rest of my drainage pipes. I was told I can use regular pvc cement with this flex pipe, so it really shouldn't be any different than bending regular schedule 40. And the rest of my PVC is white, so it fits right in. Black isn't really used around here.
This will be my one roof vent in the whole system. When it's all finished, I'll have two sinks and a shower draining into a grey tank. No black tank. I'll be using a Hepvo valve on all the drains so this will primarily be to vent the grey tank.
Flexible pvc coming out the bottom, not trimmed yet.
Pipe going out the roof vent, which clearly needs a new gasket.