As the previous poster suggests, it is not clear shether your question relates to the city water connection or to the fill location for the fresh water tank. If its for the fresh water tank, on my 63 Tradewind there is an aluminum door leading to an aluminum box in which there is a threaded neck. The cap with the valve stem is on the top of the threaded neck and a galvanized pipe is screwed into this neck from the bottom. To convert over to PVC the galvanized pipe needed to be removed. That required using a long bladed reciprocating saw to carefully cut through the pipe in three locations without cutting through the threaded neck. Then, with the correct size threaded PVC fitting the process of connecting from the fill to the storage tank could begin. From the back it looks like this.
From the front it looks like this.
With the original cap back on it pretty much looks like it did when I started.
You may notice an extra fitting in the upper right hand corner. That is an addition for a 1/2 inch line to the top of the storage tank that will allow air to escape as the tank fills.