We have a Theford Curve porta potty for our TC. It's 5 gallons. I had a camper just like you and now that I have used the Curve for one season, I'd make the black tank into grey and use a porta potty. Plus the Curve has hold down brackets...
I have small TC and the black tank was only 5 gallons anyway. So the porta potty route is great. Plus, no stinky slinky's to carry. just swing out the spout on the porta potty and dump.
We like to travel fast and stop overnight in rest areas so having a gray tank would be nice. Right now we have to attach a portable Swiss water bag when we stop.
We do the same thing you want to - remove the porta potty when we shower. Works great.