Find the main water feed line from the tank to the pump....its a short one normally. That line probably will split to two valves...one of those valves will be the drain.
(Remember now, I am the one who flooded my trailer last week with those same two valves)
"If it can't be reduced, reused, repaired, rebuilt, refurbished, refinished, resold, recycled or composted
then it should be restricted, redesigned or removed from production."