Master Drain from TM to Gray Water Tank?

While installing the power twin leveling system on the rear of my '06 2720sl, I nearly had to make a modification to the square plastic access cover that protrudes down for a few inches from the bottom of the TM near the outdoor shower box area. The cover bottom has a round hole cut into it with a type of screen material covering the hole. There are two drain petcocks right next to the cover.

After taking the cover off it appears to be (my guess) the master drain for all TM plumbing that leads into the gray water tank. It comes down from the TM floor, leads into an elbow or trap and goes back up and looks like it feeds into the gray water tank.

No real question here other than is my suspicion correct? Luckily, I didn't need to modify anything as the access cover is flexible enough to not give me any problems.

Dano