I added the extra valve and do merge tanks at times. Because it twists on just like the cover I drilled a small hole and added a small screw dipped in sealant where it attaches so it cannot rotate.
Reminds me of the time with our old TT. Had a site with electric & water, no sewer hook up. After a day or 2 I thought I'd drain the gray tank into a blue tank. Tank filled quickly, along with the 10' hose . . . . fortunately it was only water and no "stuff". We now have a 337 with 2 gray tanks a the gate valve so we can combine the tanks and camp for a week . . . .