It is usually below or near the outside gravity fill port for the freshwater tank. Those fill ports are in close proximity to the holding tank which is also near the drain tube.
Sometimes they put the valve inside an exterior compartment by accessing a small door or panel near the water pump. Locate your water pump and look for a valve.
Sometimes the 2 low point drains also incorporate the freshwater drain, When you open the H/C low point drains, if the cold line seems like its taking a lot longer to drain, then you found it.
Low point drains could be inside a removable panel under the fridge or under a cabinet. Some units have the low point drain exposed under the rv.