A couple of points on your holding tank and vent lines to consider,
Two kinds of bacteria inhabit holding tanks: aerobic and anaerobic. The former require oxygen to exist and reproduce; without it, theyíre goners. Aerobic bacteria break down organic matter, creating as a byproduct carbon dioxide, which is odorless. Conversely, anaerobic bacteria thrive in a low- or no-oxygen environment. The by product they create is a variety of gases, including sulfur monoxide and sulfur dioxide. which impart the pungent odor (and methane and carbon dioxide, which are odorless).
1) In order to provide adequate airflow, vents should be larger than the typical 5/8-inch hose. Ideally, the inside diameter should be a full inch;
2) Vents should also be plumbed to opposite sides of the vessel, enabling cross flow, in a direct line with as few bends and dips as possible.
3) Remember, the aerobic bacteria, which digest waste and donít produce odor in the process, require oxygen. So providing it is often the most effective and simplest approach.
4) If passive cross ventilation canít be achieved, a holding tank aeration system is a viable alternative. (Like the Sani Tank System)
5)The larger the exterior vent can be, the better.