OP, here . . . I bought a portable unit. Yes, this is a clip-on. I won't have a chance to try it for a while, but hookup for both the macerator and the rinse water are ultra easy. I even have a SAE 12v source on the same side. Really minimal hookup difference from the gravity option. I was also surprised how small the unit is.
Part of the problem for me is storage of a 3" gravity hose. This depends on make of the van as well as the actual model, of course. I can easily store an extra garden hose as well as an add-on macerator. And, like pointed out earlier, if I have a problem I do have a gravity backup.
Another use, even if I choose not to use it at the site, is that it will be perfect to empty the tank into our home septic system.
Again, thanks for the replies. This site a great source of information!!
EDIT: PS, when we were shopping, we were really taken with the rear-bath floor plan. This limits selection a lot, but still it was our preferred arrangement. This arrangement tends to separate the grey and black tanks by quite a distance, so I am not sure if there even is a brand that uses a macerator with a rear bath.