Not sure with that camper, but EG, in our last two older ones, no grey tank either. Idea was dump the grey into a ditch with a hose, or put it in the black tank till you dumped that.
Our camper is long enough that it hangs out the back of the 8ft truck box. I got one of those blue tote sani-tanks and hung it upside down there supported underneath by a ball hitch with no ball, out of the hitch receiver.
I cut a hole in the bottom (now on top) and have a short hose that runs from the sani-cap with the hose connection to it, to the tote. The hose has to be fat enough to let water out as fast as your sink pours it in, or it will come up from the shower drain.
Ours has a grey tank valve, but the OP would have to add the valve somehow so he can isolate the black and dump the grey. Just leave it all open so when you use water in the RV sinks, it goes down into the blue tote (spray painted to the truck's colour)
When you can, as in your yard at home (or whenever you can get away with it), to dump the grey tote tank, just put a bucket under it and undo the main screw top, now on the bottom. If you do that on a rainy day, the rain washes it all away so there is no smell. :)