Assuming your HW277 is similar to our HW296, the black holding tank is above the floor, underneath the toilet, so it is not visible from underneath. The other two tanks you see connected together are the gray water tanks, 6 gallons each. To drain the gray tanks into a sewer connection, you will need an adaptor. Some people go from the 2" fitting up to the standard 3", while other reduce it down to 3/4" garden hose size. I do the latter with a Valterra T010091VP adaptor fitting at the trailer end, and a Camco 39111 adaptor at the sewer end.
There are no indicators for the level in the tanks. The black is pretty easy, since the toilet dumps directly into it. Just open the toilet valve, and you can see how full the tank is. The gray tank is a little trickier, but one indication is, when the tanks are past full, they will back-up into the shower. Since it is just sink and shower water, it doesn't hurt anything.
Bob & Doris
2009 Toyota Tundra CrewMax 4x4 - 5.7L - 6 Speed Auto - 4.30:1
2013 Rockwood HW296