Another thought, it may not be your connection. You could have crack somewhere in the pipes or a leak in the swivel (if there is one). If there was no leak on the up side of the blade valve then at least you could isolate it. You could check it by putting nice clean water in the grey and only opening that one.
I am sure you can find pictures of your same unit for sale somewhere and see a pic of the setup. Here is one with a video and pic 19 shows the wet bay.
39zMessing with this is one of the reasons I installed a macerator. I put it on a remote switch. I hook the hose to the drain at the campsite and leave the valves open when camping (the macerator requires it to be running to open the flow) and when the tanks are full, I simply hit the remote FOB from the comfort of a seat inside and it dumps itself. I have a bypass installed in case something goes wrong and I need to do it manually (which I haven't in 2 years) I also have flush kits mounted on both tanks.