How to empty tanks on a Palomino Solair 267BHSK

I have the same trailer, and the forward grey tank is plumbed into the pipe ahead of the black tank valve so you have to open the black valve then the grey tank valve for it to drain. Its not a problem as it helps flush the black valve and piping as well as a vacuum on black tank. I use my trailer once a month and it works great. The user manuals are very generic at best. "On the job training" is the best bet!