The tank drains are usally a red hose bib type handle that is connected to a brass valve. In your model you should have a black hose poking thru the belly pan in the middle of the trailer. Above this hose there will be a valve handle. This will often be under a kitchen cabinet poking thru the floor. Open it up without the pump on and water in the tank and you should get water out of the drain tube. I suggest if you find more than 1 valve you open them one at a time so you know what is what.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato