my advice is to open your cold and hot water taps at kitchen sink, then power up your water pump and allow it to push the stored water through piping until supply is exhausted. opening the hot water tap should also allow for any water in hot water tank to be push out as well.
1975 31' Sovereign
1973 27' Overlander
1977 23' Safari
Join Date: Oct 2005
Some late 70s models hide the drain up above a roughly 12x12 steel plate in the center of the tank, out in the center of the belly pan. This is a true unbelievable stupidity. My 77 Safari is like that. You take the plate down by removing a large bolt in the center of the plate.
If you've got shore power, it's probably easier to empty the tank using the pump , per JGSIMONS suggestion.