[JRTJH]

According to the Keystone website, your 2006 Montana 3400RL has one 60 gallon fresh water tank, and 38 gallon black tank and 76 gallon gray capacity. I'd suspect that the 76 gallons is actually two 38 gallon tanks, one for the bath sink and shower and one for the galley. My guess is that the handle you've found is in the open position and the galley tank has been draining freely. Keystone regularly uses the same size tank for black and gray (and makes double the gray capacity by using two tanks).

You didn't mention how you camp, so if you're always connected to sewer when you use the trailer, you wouldn't notice any issues with the valve open and the trailer connected to a sewer. If, however, you use your trailer "dry camping", have you noticed a "gush of gray water" when you go to the dump station and remove the cap from the sewer connection?

My thoughts: The two valve handles in the garage are the bathroom gray and black tank handles and the one you found under the trailer is the galley gray tank handle.

Depending on how long you've had your trailer and how long that valve has been "unused", you may find that it's "frozen" in position and won't slide to open/close properly. A careful examination without forcing the valve handle would be your best option. If it won't move, don't try to "make it work" with force !!!!!