I'm not familiar with the 4700- is that a typo or something different from a 4500?
The cap part of those hatches isn't replaceable- you have to replace the whole thing, which means drilling out the rivets, pulling the unit out, disconnecting/reconnecting the attached hoses, and reattaching.
Per the gray water drain- my '78 4500 never had a swing-type lid...I can't see enough detail in your pic to tell if yours is the same as mine. The "keep cover in place" decal actually refers to a screw-on cap like this:
The purpose is to keep sink water from drooling down the side of the trailer when in motion.
I wanted a hinged cover over mine, but couldn't find anything remotely resembling the connection in the original color. I did find electrical
hatch covers in the ivory, so I cut out the innards of a new one and installed it directly over the old one. This also saved me all the trouble described above...