I recently replaced the entire valve assembly on my 7-year-old Class C just to avoid an experience like the OP's. The handle on the grey tank dump valve was cracking and that was all the warning I needed to motivate me to take action. Even with regular use and maintenance components like these can and eventually will fail and usually at a less than convenient time and or location.
As I recall it was around $40 and only took about an hour working in my driveway with access to all of my tools. When I posted about it I got about 3 pages of "If it ain't broke don't fix it" and "preventative maintenance is a waste of time". I continue to disagree. :R
Not sure how the OP will be able to reconnect without pulling the entire unit for disassembly and any temporary fix is going to be just that. If you have to remove the entire unit I recommend just replacing it. You can find a replacement valve assembly on Amazon. Good luck!
:C
