Boiled down, you want to install them as close to the tank as you can and in a place that won't get any water in it. The fresh water tank
vent can be in the cabin
(mine is). Just put it some place where if you overfill you'll see the water start to gush out. If you want to be fancy you could have it drain into the sink or something, but really anything will do. The vent shouldn't be used as a method of determining tank fullness.
The holding tank
needs a big vent; two if you can make it work so that there's a cross ventilation going on. I'd put them through the hull
topsides (vertical surface, not on deck) with a clamshell facing mostly down but slightly forward on one side and a clamshell facing mostly down but slightly aft on the other.
vent you need to be careful with because if you get a bunch of seawater or even rain into your tank it can really muck things up far more than the other tanks. Mine's located in the cockpit
underneath an overhang of sorts, aimed down. I look at it suspiciously because a pooped cockpit
would put *a lot* of water into the tank.