Drain valves on fresh water tank leak at tank

[MG1234]

I have 2 large fresh water tanks that are tied together under my toy hauler with 1-1/2 rigid pipe tapped into the bottom of each tank.over the years the bottom of the tanks have sagged a little causing stress on the fittings on the bottom of the tanks ,and now they drip. I'll cut out the rigid pipe and replace it with flex pipe but I need to understand how the 1-1/2" taps are made into the bottom of the tanks so I can repair those .they look like a hole was drilled and a rubber pipe was glued in ,but that doesn't seem practical,anyone know how this is done when trailers are built?Mine is an 04, thank you ,