The 100 gallon water tank is designed to be installed with the large threaded inspection port (lid) at the top for filling, inspection, and cleaning. This port is not intended to be placed on the bottom, as it is not a sealed outlet and could leak if oriented downward. The outlet with the red plastic piece is intended as the main water outlet, not as a vent, so using it as a vent would require orienting the tank differently than designed and is not recommended. For venting, it is best to a dedicated vent fitting or consult your installer to add a proper vent in the correct location. Regarding outlet fittings, common options include 1-1/2" or 1" threaded or barbed fittings compatible with your plumbing setup, such as a 1-1/2" flex rubber grommet tank fitting that fits a 2-1/4" hole, allowing secure pipe connections. For more details, see the product page for the 100 Gallon RV Fresh Water Tank 81" x 26" x 11" (SKU: T100).