I see an allen screw in the middle of what looks like a rubber stopper on top. If you remove the allen screw do you have access to inside the tank? A picture of the tank is shown below on the WM site. Thx http://www.westmarine.com/buy/seala...KefCdr2DTZtsAhVWP_8rD1m6e3Z8ZcKuRbRoCkD_w_wcB
If you don't have one, SeaLand's manuals are available on their web site. http://www.dometicsanitation.com/International/Documents/Manuals/ Like half the world it seems, they are owned by Dometic.
I believe that to be an over pressure valve and will not offer you anything. You need to get in the tank?
Most of the other fittings are pressed in. It's one heck of a gasket but the top 1.5 inch PVC tubes are just pressed in from the top, lots of wiggling and upward pulling, it is lots of work but they will come some day. If the picture you offered is the exact one in your boat (there are many models) then pull anything from the top that would be considered in-flow. You would want the out flow pipes left in to pump out your rinse. Now, the big question,, Why?? If your thinking your going to clean the stain on the inside of the tank, your in for disappointment. That roto-cast plastic is NOT smooth as the outside. It holds the grime like non-skid. Raritan K O is your best cleaner and odor eater. If you think you see mud building up on the bottom, you probably do. After hours of trying to rinse it out, it will still be there. Don't worry about it unless you really have a vision (you see it) problem then just install another tank.
I agree, you are not going to get in your holding tank and clean it, nor should you need to. The raritan additive is a good option. Another option is to empty the tank, then flush 1/2 a cup of HE laundry detergent liquid down each toilet, let sit for a few mins, then fill the bowl up and flush several times, and then get the tank to 3/4 and let it sit overnight, pump it out......Sealand came out with a tank cleaner.....I'm working on the boat where his "broker" talked him into a guy cleaning it.....it was clean for about the first time it was filled to 3/4, after pumping it out, it is as stained as it always was LOLOLOL.........