Trying this one
Since I didn't receive any responses I'm going out on a limb and using 3m 4000 UV which is marketed for boat owners. There's a little bit of rust action starting in this area which it sounds like the older vans can get. Beneath the temporary top layers of caulk that had been applied by who-knows-who (we bought this RV used) there was a yellow, strong, partially-crumbling gap filler which still had some elasticity since you could dent it with a fingernail just a little bit. I'll just assume it's a urethane caulk of some sort. I'm not sure if the 3m 4000 will stick to that but it is extremely gooey and sticky out of the tube so I'm hopeful. If not it can be removed.
My local RV repair shop receptionist lady recommended Dicor non-sag formula for vertical seam sealing. I purchased a tube and didn't like the fact that you could put permanent indentations in the cured Dicor with just a fingernail. The 3M 4000 cures more like a dense rubber but not as shiny as silicones. The Dicor seemed weak to me. I also bought a tube of Proflex RV, which looked and felt more like a typical silicone (althought it's supposedly different). The 3M 4000 looked like it would be more difficult to remove and I'm hopeful that this translates into greater longevity.