I just towed my camper mounted on a Trailex with 8″ wheels on it’s first trip. Highway speeds up to 75mph, no problems. Towed it on some fairly seriously rutted and washed out dirt roads — did not need 4WD though — but enough that it bounced the trailer around pretty good. No problems other than discovering that I need a better system of securing things in the galley. Tires threw an impressive amount of mud though, the fenders did their job well. Even for “normal” highway towing, you *will* want fenders.
I chose 8″ wheels specifically to *lower* the sitting height and the ergonomics of the door sill height as well as the functional galley flat height. On 12″ tires I think I would want a little step to comfortably use the galley , in fact CLC has just such a step they use with their demo TD. On 15″ tires the galley would be almost up to my chin!