Chris, I had to build mine after I bought the trailer. There maybe a diagram or at least I know there are parts listed in the UHaul
Document Center of this Website.
I am suggeting that you Print out the Manual and the parts Manual for your personal reference.
The graywater from the sink drains directly down and through the floor out. Should be caught in a bucket under your trailer while camping. For this hose, I used "Bildge tubing" It's for boats. Or You can buy the whole thing at an RV supply store.
You can find all the hoses and tubes in a good hardware store. Throw the old ones away, not worth cleaning. Then clamp the new ones on. Save the Flow Valve (tiny plastic thing) and connect the new tubes to it, making sure it is facing the right way. (It has an arrow on it.) This could be what you are thinking of as a vent. The Fresh water tank drains out the back of the trailer throught a "pet cock" This is a $2.00 item that can be bought at any RV supply place.
I bought all the tubes at a True Value Hardware store and cut them to fit. The measurements are in the UHaul
I remember the most difficult thing was connecting the short tube in the back of the tank to the drain cock as there is not any room to manuver. You are a big guy, may want tiny woman fingers to do this. HA! And I had to hairdryer heat the tube connecting to the pump faucet to get it to fit, this tubing is thick. You can rebuild it,
Piece of cake!