I agree that Thetford's documentation
leaves a LOT to be desired!
Which version did you buy--the 550P, or the 550P MSD? The MSD version has fittings for a pumpout line and vent line...the 550P (no "MSD" in the model name/#) does not.
If you bought the non-MSD version, you'll have to replace the pour out spout with a PVC pipe that goes to within 1/2" of the bottom of the tank..you'll connect the pumpout line to it. If you only plan to dump the tank at sea, you can install a macerator pump in the pumpout line and route
it directly to a thru-hull instead of a deck
pumpout fitting. If you also want to be able to pump out the tank, you'll need a y-valve in that that line ahead of the the macerator. You'll also have to install a fitting (or something you can connect a hose to) in the top of the tank for a vent line.
Sounds a bit complicated, but if you're careful and have help from someone who knows what they're doing, it'll work
Or...you can return the 550P and exchange it for the MSD version. Since the boat
is still in the process of being built, it doesn't sound like you'll need it for a while and it'll be very easy to install and plumb once you have the version that's meant to be pumped and or dumped.