Venting for Black Water Tank...

Crazymom… here is the best picture I have on hand of my roof and venting: s1281.photobucket.com/user/sligobeach2/media/DSC03077_zpse70816fa.jpg.html

To the right, towards the front end of the trailer, is the main skylight w/ vent cover.  Behind that (out of view) is a passive vent over the stove… to the left is the propane fridge vent… to the far left is a vent for the Vaillant on demand propane water heater.  My proposed black tank vent would pop up about a foot and a half to the left of that, where the rear curve begins.  My black tank runs nearly the full 7' width of the trailer and is about 30" the other way and rather shallow (and sloped toward the exit) since it is mounted to some steel cross members under the trailer floor and is exposed to the road.  It's about 20 gallons total.  It's pretty large for a trailer occupied by 1 or 2 people.  So like any household with a septic system on property, size it based on number of users and expected duration between dump station visits.  I've been told that the original black tank on this rig simply had an air vent out the side of the tank consisting of a 1/2" copper pipe and about a 1" square vent at the bottom of the side wall… both of which are still there.  It allowed the tank to respirate when flushing and emptying, but probably made for some interesting smells when camped.  Getting that respiration to happen above your head and not below your feet, is preferred according to most on this forum and others I've consulted.

There is huge debate on other RV forums as to the wisdom in combined tanks.  Since a black tank has finite capacity, unlike your city sewer line, there is potential in some situations for back up into low plumbing fixtures like shower drains, etc. if the tank becomes too full.  If you are able to carefully monitor your tank, you can avert that particular bit of hell.  But I think in your case it's moot.  On Airforums, the Airstream users site, they frequently point out that the elite Airstreams had combined gray/black tanks until the mid 70's.  It was very acceptable to drop gray water on permeable ground up until then and in my opinion, it still is… as long as your gray water is indeed just gray and not full of chemicals, detergents, food waste, etc.  In your case, with just a kitchen sink, and a good drain basket, don't bother with a gray tank.  You can either dedicate a hose with attachment and run the water into the bushes to be discreet, or if the campground is strict, carry one of those small blue portable tanks and wheel it to the dump station before leaving (or as some suggest, loosen the drain on it slightly, overnight and let it dribble out… by morning it will be safely absorbed back in to mother earth, where it all came from in the first place ;-)…. to be further discreet).

Let us know what you decide...