I believe you have it just right. On trailers with the black tank mounted above the floor, like ours, you cannot back grey water into the black tank, it will seep into the shower first. We have (too many times) forgotten to open the vent on the external tank and messed up the shower when water backs into it. But that is what you will need, a 'blue boy' external tank (they come in grey too). You run a hose from the sewer outlet to the tank and the grey water will run into it. Then you dump the tank at the dump station when you empty the black tank.
You can get away with a couple handwashes when you are traveling, and the water will back up in the pipes just fine, but any more than that and it will show up in the shower.
BTW, congrats on snagging a Caravel
- I am biased, but I think they are the best small trailer out there, with the best layout. We love ours!