Congratulations on he purchase of a 75 Trade Wind, my favorite!
The Black water (toilet) is the Main tank and the valve for that tank is located inside the rear bumper in the middle.
The Grey water (sink & shower) is the Aux tank and the valve is located next to the sewer line connector on the street side of your trailer.
Always dump the black first and only when the tank is getting full, if possible. A fuller tank will carry the wastes out better then a partially full tank. We always dump a few gallons of water down the toilet after it has drained to make sure there are no objects left behind. The grey tank is drained next which will help rinse out the sewer hose.
The tanks are small, around 10-12 gallons each and fill quickly and the gauges are not always reliable. You can visually see the level of the black tank with a flashlight
. You can not see the level of the grey tank, but if water backs into the shower, it is full.
Have a great first voyage and be prepared to make a few mistakes along the way. We have all been there...