No you cant screw up draining gray tanks. That is as long as you pull the handle and drain the water. My guess is mislabling. You might buy yourself a lable maker and once home go around and figure out what everything is/does and make appropriate lables for them. On my fiver I have three valves, all terminating in one place. So the lables are rather simple. Gray,gray,black. Simple deduction on my part let me know the valve over the tires is kitchen. Your it sounds like is not that simple, thus the need to correctly mark.
BTW, for your information, it is perfectly safe and acceptable to leave the gray valves open all the time. Been doing this for years with no problem. I usually close the gray tanks a,day before I schedule to dump the black to give me some water to flush the sewer hose.
Also, since you have two terminations they sell a Y so you can attach everything at once to your sewer dump. Might want to invest is the stuff necessary to do that. Saves the headache of moving the 4 inch hose around.
