Not sure about yours, but most all campers have one place to hook the sewer hose, and then two handles to pull, one for gray water, and one for black water. Does your camper have two holding tanks along with a fresh water tank? I can't find anything on tank size on Forrest River web site. Are you sure someone didn't put an adapter on like in post #4?
Before this gets out of hand, there are some campers, with two gray tanks, but they are usually over 25 feet, with a bath near one end, and a kitchen near the other.
Since your total length is only 12 foot, not much room for that.
2012 Ford F250 ext. Cab 4x4
2002 Jeep Wrangler Sahara
2008 Yamaha V-Star 650 Classic
2008 Work and Play 18LT