Thanks for the answers. I realized after looking at the Uniseal instructions more closely that the seal needs to go in the tank first, and then the pipe in the seal. That (as you point out, Peggie) prevents inserting the dip tube with the elbow already glued in place.
My concern with the external pipe was that it would be more difficult to get the last 1 1/2" or so of waste out of the tank, but I think that it's actually no different from what I have now, with hose rising to the Y valve (which goes to the overboard pump
discharge). So that's what I'm going to do.
I hadn't thought about using Schedule 80, but as I'm planning on using pipe where I can (where I can allow movement to prevent cracking), the heavier the better.
I *still* haven't figured out the best way to vent--I have about a 4' rise from the top of the tank to the deck
, and can only go aft from the tank in order for the vent hose to be hidden.