dbrock,
From what you describe, "appears to be connected to the boat", I would guess you have the "Dockside Pumpout" option. If there are 2 hoses exiting out the backside of the portapotti, one being 1 1/2" and the other being 5/8", then that's what you have. This setup basically renders your portapotti anything but portable.
There should be a deck plate somewhere on the deck that looks like a fuel fill, but is labeled "waste". In order to empty the tank, you need to stop by a marina with a pumpout facility. You really DON'T want to disconnect those hoses.
If this is not the case, simply lift the 2 handles on each side of the potti, pull out the white handle in front, which will both seal the tank and release it from the toilet seat part. Then do with it what you will!