I can’t remember if this has been brought up before but here goes:
From first hand experience, the best practice is to connect one capped waste hose to the drain pipe in the bumper and leave it connected. Both waste valves are a good distance from the cap on the end of the drain pipe in the bumper. If black or grey water leaks past the waste valves you will know it when you pull the hose out of the bumper instead of when you uncap the drain pipe. You can then put one of these
on the end of the hose by raising the capped end up, then add your extra hose downstream of the twist on valve. This will give you a way to release what has leaked past the waste valve without creating a mess. Even if you try to add the extra hose on after detecting fluid in the first hose, you risk creating a mess when you eventually have to lower the end of the hose to place it on the sewer connection on the ground. I now carry a twist on valve for this reason.
Bill