location of waste holding tank?

[sea flower]

I have an 83 Sea Ray SRV 260 Express Cruiser. I've had it for 4 years and have been bringing it back to life slowly but surely. I can't open the waste cap and I don't have a maserator pump. Where is the holding tank?? The head is a porta potty, original with the boat. Done the Sea Ray archives that's how I got alot of info but none that show the schematic layout. It's to old according to Sea Ray. Any help out there?? I would like to use the tank but am afraid too if there's a problem know what I mean?? Tanks!!