On our last trip I lost the OEM dumper cap....my bad, not secured properly.
This ended up being the 2hr Camping World on the road fix.
After insisting that the covers they had would fit, they didn't, we tried them all. I showed them that the spare end of a dump extension hose I had did fit. (the adaptor that FA mentioned, the white part in the photo) The the blue cap they had in stock fit tightly over that, I used Marine-Tex to bond them together.
Look for a hose adaptor that has just two "hooks" as opposed to the AS's four, that is the only style that will fit ours.
Hope this helps,
"Any nation that can survive what we have lately in the way of government, is on the high road to permanent glory."
"Mary-Jane....'cuz 2020 sucks"