They're called low point drains, and your 77 HR probably has two, and maybe three, depending on its size. At the least there should be one near the fresh water tank which is usually under the bed; and another one near the water pump, which is usually not too far from the tank.
As mentioned above, an easy way to locate how many and where is by looking under the trailer. If you see a short tube poking down through the bottom, that's a low point drain and the valve will be directly above that tube.
curmudgeon: noun; a crusty, ill-tempered, irascible, cantankerous old person . . . .
79 27' Holiday Rambler Statesman, 78 32' HR Imperial, 85 36' HR Imperial 5th Wheel