Where is H20 drain plug located, how to purify?

I use about a cup of bleach to 20-30 gallons of water in the tank. Run it through till you can smell bleach at the faucets. Let it sit a few hours or overnight. Drain, refill, reflush.
You are trying to clean the system, so you need a significant amount of bleach. If you just wanted to make the water in the tank safe to drink, then a few drops per gallon will work.

My Allegro was comletely different than your, but you can find all kinds of hepl over at the Tiffin RV Network, the Tiffin owners web site.

Can you see your water tank? It is going to be someplace below the bottom of the tank. With my motorhome in the plumbing compartment you can see the drain pipe coming from the water tank with a valve on it and an elbow with the drain pipe going out the bottom.