I had a leak in the same spot on my 2006 Flagstaff. I pulled the front trim off and cleaned it real well. I took Eternabond and wraps the corner with it and then put the trim piece back on. The Eternabond was slightly visible on the side but, no more than caulk. I think too a thin clean bead of caulk and ran it down the side.
Proflex is a good caulk and you can find it at any RV store. I've also had good luck with QUAD OSI Caulk which you can get from Home Depot or Lowes. My RV dealer actually recommended it to me as it has the same ingredients if you will than the Proflex but, it's 1/3 of the cost.
Dicor Lap sealant is for the roof typically as it's self leveling so it would possibly run all over if used on a vertical surface.
Also X2 on the sidewall, if they would make it 1/4 longer it would have been perfect.
2012 Keystone Sprinter
Still sticking around for the good people and advice.