Gray water drainage design help (see drawing)

So far as I know, the bayonet fittings are all interoperable. Some have two prongs/knobs and some have four; the four prong/knob ones are a little more secure, as would be expected. Naturally, the larger the tank the more convenient it is (within reason), but practical matters like available space usually dictate the possible size, at least to some extent.

There's nothing dangerous with having a smaller gray water tank than fresh water tank. It just means the gray water will fill up before the fresh water goes empty. 18 gallons should be enough to last a few days if you conserve water usage a little (navy showers, conservative dishwashing water usage or using disposable dishes/cutlery, etc.). If you happen to be in an area where the water supply is not potable or is safe but not appetizing for whatever reason it's nice to have a comparatively large fresh water tank. Do ensure that your shower pan has a reasonable threshhold on it so that you won't flood out the entire van if you accidentally start to overfill the tank. (The shower drain, being the lowest opening in the system, is where it would back up first.)