I have a 2006 Fleetwood Highlander Sequoya so let me speak from experience.
Yes, there is a 6 gallon black holding tank. A real one with real 4" bayonet fitted drain outlet.
You are correct in that the gray water line also terminates at this same outlet and there is no gray water holding tank. Your blue tote IS the gray water holding tank.
When setting up at camp, pull your blue tote around and connect it via typical sewer hose to the outlet and open the blade valve for the smaller gray line that feeds into the outlet. That gray line is supplied by the kitchen sink and the shower drain. Just leave the blade valve open all of the time until it comes time to dump your blue tote. Close the valve and take the tote somewhere appropriate for dumping. While that is gone, the sink can be used for a minimal amount but not recommended. The plumbing holds about half a gallon before it will start to come up through the floor drain of the shower. Yes, I've seen it happen.
When you're ready to dump the black tank, you have two options. 1) unhook and take down the PUP and tow it to the dump station and dump like a regular TT does or 2) use the blue tote or other container to drain the black holding tank and take that to the dump station.
I just dump on the day we're leaving just like everyone else with a TT. My process is much faster than most others because I don't have a large black tank and I don't have a gray water tank to dump. Cleaning out the black holding tank is challenging and different because you cannot access it when dumping as the roof is down and the folding bathroom walls block access to it anyway. I just use the water hose and spray up in there gently and catch all of the run off with the sewer hose held under the outlet.
Now the rub for me that really motivated me to make a change up was that I do not want my blue tote being filled with black water. It's a whole different ballgame on how I might handle something that contains gray water (sink water 99%, shower 1% of the time) versus black water. A very small amount of black water would dribble out of the holding tank during camping and get into the blue tote. No thanks. Even if it was sealed up tightly, there would be residual gunk on the outlet from previous black tank dumpings that I didn't want to connect the blue tote back up to.
I cut the gray water line off of the black water outlet, capped it off with a rubber cap and hose clamp. I then installed a second 4" bayonet outlet right next to the black water outlet and ran the gray water lines to it. I carry two sewer hoses now: one standard 20' length for dumping the black water at the dump station and one 10" length for connecting the gray outlet to the blue tote that sits directly under it.
It works like a champ and eliminates cross contamination. I will include a picture. The outlet on the right is the new one I added. I had to get a few fittings from Lowes to make the transitions from 2.5" dia gray to the 4" outlet but it was straightforward.
In between the outlets, you can barely see the silver hose clamp holding the rubber cap on where the gray line originally joined into the 4" outlet.
Here's another view so you can see what is going on with the plumbing.
Original sizes can be viewed
here.This works but it's not as good as it should have been from Fleetwood. My opinion is that they should have gone with two holding tanks like typical TTs have or they should have gone with a cassette potty and no holding tank like typical PUPs. This half of one, half of the other business is quite bothersome. Fleetwood got the picture as I don't think they did it this way in the follow model years. It is my understanding for what I've read of other Highlander owners that they have gray holding tanks. Adding one is on my way-down-the-road list of things to do but I doubt it will ever happen. There's just not enough clear area under the subflooring for a tank of any sort.
Also, because the outlet is so low to the ground, I decided to get the Barker 15 gallon Tote-Along made for folding trailers. It is low profile and does the trick. Since then, I've done an axle over/under conversion and have an additional 6" of clearance (I HIGHLY recommend doing this for Highlander PUPs) and could use any tote I wanted. Below is a picture of the carrier I built to store the tote under the rear bumper of the PUP.
And on "flipping" the axle, which is a misnomer, I have some before and after pictures to share.
Before:
After:
