If your 1974 is like my 1976 your black tank is ABOVE YOUR FLOOR, just below the toilet.
Remove the toilet and the aluminum and fiberboard framing around the tank.
Remove the belly pan at the rear to undo the drainage. Lift the tank out.
I replaced my 10 gallon black with a 14 gallon black BELOW the floor. I didnít like the toilet sitting up on a throne. Having a nice flat floor opens up the tiny rear bath space.
You will need to replace the plywood floor. I treated mine, which was in perfect shape, with West System Epoxy and then covered it with the same wall-to-wall Marmoleum floor covering used throughout the trailer.