I drain my tank one of two ways depending on how quickly I need it to drain.
1) Quick and dirty, run the faucet and drain through the sink drain. My water pump Shurflo 2088-403-144 is rated for around 2gpm and I have a 20 gallon fresh water tank. It can easily handle 2 five minute continuous runs without overheating. So I can have my tank drained in about 15 mins or less that way. Now, I don't have a grey water tank so the sink drain simply drains to the ground. If I had a grey water tank those usually have a pretty large drain valve so I would have to drain that as well but those drain pretty quick too.
2) Slow and steady, my fresh tank has a drain valve just like yours, however, I also have a valve that looks like this attached to the tank.
That grey valve is an inlet shutoff valve. Pretty standard on SMB builds. If I close this valve, I can go connect up my hose and turn my house spigot on, then get inside the van and open that valve and start filling. Once full(overflow is draining to the ground) I close that valve and then go shut off my house spigot. If you have that valve, I would open it while draining. The other connection to my fresh tank is a fill port that allows me to fill my tank directly from the inside of my van. I also open that port when draining.(2nd pic)
It definitely takes longer using method 2, but I have a solid stream of water coming out of the drain the entire time when I use method 2.