Ok, here's what you "should" have:
The toilet (black tank) and bathroom sink/shower (front grey tank) are the pull valves that are in your wet-bay.
The kitchen sink runs to your 2nd grey tank... and the handle for that is probably near your rear wheels on the driver's side (street side), as shown in these photos:
When you dump your tanks... ALWAYS dump the black tank first. Then dump your front grey tank, as that allows you to fully wash away the sewage from your black tank thru your hose. Then dump the 2nd grey tank by pulling the handle by the tires.
Now you also have a fresh water tank dump valve... and it's probably located somewhere underneath the center of the trailer, and looks like this: