A couple of years ago (when I had a 34-ft LY diesel moho) I had the same thing happen. At the street water entry there is a check valve. Unfortunately, AS used an all-in-one assembly which is almost impossible to get. However, I was able to get a street water inlet with a checkvalve that just had an aluminum flange. Using a Dremel tool with a variety of cut-off grinding wheels and bits, I removed the street water portion and substituted the one wiith the aluminum flange (had to trim the flange, again using the Dremel tool, to fit so that the plastic door would close). Some of the fittings to the inside this street water inlet may have to be modified. In my case, a flexible hose for a faucet had the right ends - the hose was covered with woven stainless steel, so I was confident it would withstand the street pressure.
As a temporary fix you could screw the plastic plug into the street water fitting.
1987 Excella 32-foot
1999 Dodge Ram 2500HD Diesel