New Member with Problem, Underbelly Access

[JimSell]

Been camping forever, 70 years old. Went from tent to popup to a 2004 Keystone Outback Super-Lite 23RS in very good condition. Bought it last week and the prior owner did not tell me the black tank drain does not work. Gray tank drains fine. T-handle for the black was not connected to anything, black tank is 1/4 full. How do know how the tanks are laid out under the trailer? The bottom is covered with a black plastic sheet with bolts and washers along the sides. I guess I will have to jack up the trailer and support it in order to get underneath. The black tank T-handle is 36" long so I assume the valve is probably on the door side of the trailer, the opposit side from the outlet drain? What is the best way to open the bottom? Try to unbolt it from the side or cut through the plastic and duct tape it closed later? Any help would be appreciated.