Re: rv plumbing questions, really need help

One opening should be for outside vent. You could use an under sink vent and cap this opening.
One 1.5 or 2 inch is for the drain.
The tank can be any direction as long as the drain is in the bottom and it is down hill from your fixtures.

Have you though of using one of these portable tanks
You may need as much as a gallon. This could be come pricey!