You can connect the hose but don’t open the valves; it’s not a home sewer system. You want the black tank ¾ full before you dump it to wash out the solids. If you have to dump it, add water to the tank until its ¾ full. Then open the gray tank valve to wash out the residue from the black tank.
I don’t know what chemical they used in the tank, but I suspect it was to control odor. You won’t know if you need to do that until you use the TT. We’ve never gotten any odor from our black tank so I’ve never used any chemicals. I can’t answer your third question.