Occasionally, you will find a sewer connection in a campground with a threaded plug that screws in to the end of the sewer pipe sticking out of the ground. It’s a big plug and usually has a large square “top” on it so you have a place to hold while screwing it in or taking it out. These plugs can be really tight, easy to cross-thread, can be very hard to turn, and it becomes nearly impossible with your bare hands.
The plug is so large that no “normal” wrench will fit it. Many RVers carry a pair of “channel-lock” pliers that, when adjusted fully open, will fit the square top and you can use these to loosen this plug. BUT, the big channel locks are expensive.
A cheap tool that works well for this is a simple “filter” wrench—one of those with the adjustable metal band on it designed to fit around a filter on your engine. You can sometimes find these for less than one dollar in the bargain tools. They will work on the square plug just fine.
I don’t sell them. Check your local yard sales.