I just pulled mine out of my 67 tradewind-- You will have to remove the back piece of subfloor and expose it. It sits in a galvanized sheet metal pan that is probably rusted away and barely attached to the frame any longer- at least mine was--- once the top s exposed, you will have to cut the sewer pipe going out the back-- try to cut it at least 3-4 inches away from the series of 45 degree bends so you can repair the line later with PVC repair coupling.
Once it is cut, pull it out from the top and maneuver the sewer pipe through the penetration in the aluminum underbelly for the effluent. If the sheet metal pan is rusted and no longer securely connected to the frame, you will have to get a new one made-- I would take it to a sheet metal shop and have them break you one to match-- probably much cheaper than buying one from a vintage supply-- I didnt look for one though so that is just a guess. email me if you want pics of the subfloor removed and the tank exposed.