My caulking skills leave a bit to be desired... I've been worried about how to caulk the windows and the vinyl trim without making a terrible mess, especially with the large bead of caulk that will be required around some areas of the vinyl... So I decided to try masking the area to be caulked, and I'm glad I did.
I used a plastic type caulking tape for the corners... I think body shops call it fine line, as it bends easily around even tight corners. The first pic shows a strip of tape that I put on the panel right up against the window frame where I ultimately want the caulk to be. It's only there to line up the strip of tape that goes on the outside of it. I also applied tape to the face of the window flange. (see 2nd picture) After everything is taped, I removed the first strip of tape from between the other two, and now have a nice even gap for the caulk all the way around the window. (3rd pic)
The 4th pic shows the mess I made applying the caulk, and the 5th shows what the caulk looks like after the tape is removed. I think you have to pull the tape off while the caulk is still wet, or you'd end up with tape stuck under the caulk, especially this polyurethane caulk which is extremely tough.
All in all, it turned out better than I had hoped, and now I have a good method for caulking the rest of the rv. It's pretty time consuming, but I think it's worth it to gain the detail. The caulk actually looks like a cut rubber gasket around the window.