Its not unusual to have more than two tanks, especially if the bathroom is located far from the kitchen... ie Rear kitchen and center bath or the reverse. Typically your gray and black tank will exit from the same discharge point, with a value for each tank. You may also have an additional gray tank located closer to the kitchen.
Be sure to use the proper RV toilet paper, never throw wipes or Q tips in the toilet.
Also, a plunger will be useless on a black tank clog since the tank is vented to the outside, you will get no pressure build up in the tank.
Use plenty of water when dumping your black and gray tank, after it is drained, fill it and drain again, using black tank flush if you have them. Amazing how much gunk you will continue to discharge from an "empty" tank.
Tom and Linda
Tundra Double Cab
Jayco 2015 27RLS