In the course of my investigations I learned a couple more things about the weatherguard channel.thank Travlin. I’ll check with them.
The Q and A from weatherguard is here. Van Accessories. They consistently refuse to give advice on any application related to RV. Maybe liability concerns?
Their own roof racks attach to the rail with this J mount. A plate with two studs welded on slides inside the track and the J mount bolts onto that. 24-00019.pdf
Mcmaster-Carr sells a square unistrut (1 1/4 inches on a side, 1 3/4 diagonal) nut that would fit and not rotate ( but in steel only, not stainless). McMaster-Carr
also, unistrut nuts fit but the grooves don’t line up. So tightening/loosening would be problematic but perhaps workable.