Another thought... Assuming it's tied to your septic / sewer system could be a wrong assumption. The last house we lived in did have a clean-out, on city facilities. The current house we live in does not have a clean out. It's on a septic system. But, around the parameter of my home are a couple of vents with slotted caps on them. At first (several years ago), I thought, whaoo! I can dump right in them. But further investigation and living there a while, I eventually learned they were not tied to the septic at all. They were vents for the drainage system around the parameter of the house. They are not connected to my leaching system. But they are connected to the field tile in the field behind our house. Dumping my trailer tanks in there would have been a bad mistake.
Since then, we had installed a cap on our septic tank for easy maintenance and easy pumping every few years or so. Very convenient now. I can just dump my trailer contents directly into my septic tank. Since we don't use any chemicals in any of our holding tanks, except kitchen cleaning stuff, nothing there to disturb the septic tank balance. Works great for us now.
Just saying, without knowing exactly what that opening is for, proceed carefully before dumping in it and you know for sure.