Hi Audrey, glad you got a system that works for you!
Sorry I didn't see your post earlier.
I'm going to get a bit dicey here ... in my post ... about some bathroom habits ... so for those weak of spirit ... stop reading now.
Over the years, we've developed a system that works, using a 15 gallon blue tote tank, for both gray and black(modified) waste water.
The dump valves on many of our trailers are really, really low to the ground ... so the trick is often to position the blue tank 10 or 15 feet away from the trailer, and connect it with a dedicated piece of garden hose. As many have already said, replace both the trailer sewer cap and the blue-tote cap with caps with regular small hose fittings.
The trick, when dry camping
, is to not use the toilet for certain solid functions and to keep soiled toilet paper demurely in a scented, frequently-changed bag, along side the toilet.
Don't leave either the gray or black dump valve open. Fluids flow better if you dump the entire contents at the same time. Remember to "walk" the contents of the hose towards to the blue tote before disconnecting from the trailer. Lift the hose close to the trailer, then progressively closer toward to the blue tote, so the hose will be empty when you disconnect from the trailer ... otherwise you'll be in a rude surprize.
If you are going to have to dump the blue tote by "lifting" and pouring into a pit toilet (legal in some National Forest campgrounds, not legal in others, so check first), you might have to dump the tote each day (dumping first the black, carrying to the pit toilet, then returning for a load of gray), because fluids weigh 8 pounds a gallon. It's almost impossible to lift a full 15 gallon (120 pound) blue tank, so dump more often.
We also have a short piece of regular sewer hose, with fittings (one straight, one 90 degree) on both ends, for use when a -- the dump valves are high enough and b -- when the black tank contains solids that would clog a regular hose.
By the by, what you did (dumping into a dump station without attaching a sewer hose) isn't legal in most places. I know, I know, you said it was just gray water. But, most places require that you attach a sewer hose to the blue tote and then place the other end in the dump station hole. A sad fact of life is that while you may only dump gray water this way ... the next guy may dump black containing all kinds of nasty things, and while most visible signs can be washed away, the bacteria and germs aren't, and the result would be a nasty smelling mess for the next guy to step in.
As I said, the trick is to use a longer piece of hose ... and position your blue tote far enough away from your rig when you dump, so that the tote is downhill. Try to keep solids and toilet paper out of the black, if possible (hey we all have emergencies). Don't leave the valves open, so that when you dump, the fluid running through the hose acts as a syphon, emptying the tank completely).
In many situations, if the campground has a dump station, it's often easier to hook up your rig and make a short hop to the dump station. It doesn't take long to hook up our little bitty rigs! And occasionally, it's the only way to handle the contents of the black tank.