Close - those caps match the fresh water tank drain.
My caps are gray plastic and hex in shape so you can put a wrench or socket on them (which I did...no, nothing is cracked!)
No gaskets, before or after my meddling with things. The threads I would think are pipe threads - tapered - so wrapped some teflon tape and its a lot better, but still dripping.
I don't think a hose type gasket would fit in the cap. Pipe dope would do it, but i've always had to use heat to get that apart and everything's plastic.