Well if I were you, I'd get the original
head in good condition again, and leave it plumbed outside directly to the sea. That way when you are traveling, you can use it and not be putting anything in the
boat.
You can even use it at a
dock for just peeing. I know I don't have a problem peeing in the sea, but #2 is another story. Use your other
head with the 40 gallon tank to take care of that.
It's a nice option to have, a direct deposit head and one with a
holding tank. That way you've got it covered and you are
legal. That way you don't stress you tank when you don't have to.
The plus side is that you put all that
money right back in your pocket and you are already set up to hit the sea.
Best of luck.