Wow, that does stink. I did something similar once. I left the black tank valve open at the end of a trip. When I went to hook up the sewer hose at the start of the next trip, I noticed it. Thankfully the cap didn't leak. After closing the valve, I was able to catch most of what was in the plumping between the valve and the opening in the sewer hose when I removed the cap. Just a small mess.
Just an FYI if you are in a campground you aren't boondocking. Without hookups, it is dry camping. If you have water and electric it's partial hookup. Full hookup is with water, electric and sewer.
-Michael