I used the Brophy clamp on tiedowns for years with several campers and they work, but they’re a pain in the butt to have to bolt on every time you load the camper, or super ugly if you just leave them on. The other thing that you’ve already discovered is they give you very little room for your turnbuckles. They do work, but I got tired of mine.
You’re supposed to use tiedowns that are spring loaded, most turnbuckles sold as camper tiedowns are, if you use solid frame mounts. Otherwise the flexing and bouncing wild driving tries to rip your tiedown brackets out of your camper. The clamp-on Brophys have a bit of flex to them and might be OK without something spring loaded.
I can’t seem to find them anymore but there used to be a thing called a belly bar. It was a piece of square tube that went under the truck from left to right and had brackets to attach it to the frame.
Then you had little removable pieces that fit into each end to attach your turnbuckles, just like the popular Torklift frame mounts do. I have one on my truck. You could make one pretty easily.
I welded 7” of 2” square tube onto each end of my trailer hitch so I could slide smaller square tube into it to hold my rear turnbuckles. When the end pieces are removed you don’t even see it, it just looks like the hitch is wider.