Without any scientific evidence at all, I believe that's just residual water left in the pipes after dumping. For whatever reason, it didn't make it to the stinky slinky during the dump process but during the drive it migrated to the end of the pipe.
Google search for Valterra twist-on valve. It attaches to the end of your sewer pipe and adds an extra valve. You can hook up your hose before opening this valve so nothing spills on the ground or your shoes. Although some worry about this valve vibrating off during normal driving conditions, mine has been on the 5er for several years of fulltiming without glue and has never come off.