Thanks for the replies!
I think I may order some of the discharge hose so I have it on hand; the prices I'm seeing online are much better than West Marine, so I can afford to have a little left over and it will still be cheaper than running to WM once I have everything apart.
My holding tank is under the forward settee (the little "wing" on the U-shaped dinette on the right-hand side) in this picture:
The head is on the other side of the bulkhead behind the settee:
The bulkhead/wall can't be more than about 3' long. If I assume the inlet side of the tank is on the inboard end of the settee (which I'm 90% sure it isn't), and I allow 1' for each "elbow" bend plus a foot for it to go from the bulkhead to the head itself, then I'm looking at a 6' piece.
For the line connecting to the pump-out port on deck, I need approximately the length of the bulkhead (3') plus the rise to the deck, plus whatever additional length is necessary based on any "sideways" run to the fitting. In my case, the deck fitting is about where the bulkhead is, so I don't need to allow too much for the sideways traversal. I don't know the exact difference in height between the floor of the cabin (where the tank sits) and the deck. If I assume it is 6' (that's about how much headroom there is) plus the 3' for the bulkhead that's 9' for that run. I'll probably round that up to 10'.
So, so far I'm at 16' of the 1.5" tube. I think my tank may have a 2nd outlet port that goes to the mascerator. I'll need to close that off (I'm going to remove the mascerator) unless there's a valve already built into the tank. If I'm wrong and there is a Y valve, then I may keep the Y but cap off one end of it (though that seems like it may be a point-source for failure, and installing the Y again later shouldn't be that difficult, though it will likely be messy).
Then I just need the raw water intake hose and the vent hose. By my "back of the envelope" math, the vent hose needs to be about as long as the discharge hose (i.e. 10'), but I don't know what size fitting is used on the tank so I may have to wait and buy that the day I do the rest of the work.
The intake hose is under the long settee (the back portion of the dinette) on the port side, but I forget exactly where. I think it's roughly centered, but to play it safe I'll assume that, to get to the head, I'll need about 6' of that tube.
Does the raw water tube need to be the same sanitation hose as the rest of the system, or can/shoud it be something else?