I think aftermath is right. On mine, there is a white plastic valve with a handle that feels like it wouldn't take much to break off. It is the fresh water tank drain. Not far from it are two metal valves that look similar to the drain petcock on an older car radiator. Those are low point drains for the hot and cold water lines on that side of the trailer. I have another low point drain located near the water heater on the other side of the trailer. It is one of those chrome 90° knobs like many of the other internal cutoffs.
The two you spotted are pretty much standard. Any additional low point drains may depend on the layout of your trailer. Some have only those two.
Air Safe Hitch, Reese Dual Cam 800#, Centramatics