I just came back in from looking at my 2008 Starcraft 14 R/T trailer. It definitely has the gap you are referring too but under the roof lip is a section of rubber seal which comes in contact with the top metal ridge of the half door. It actually touches it for a good seal on my setup... I can force it out of the way with my finger but leaving it alone when I open the half door with the roof latched down it just pushes right on through it holding a good seal.
Having said all of this I have had a spongy floor under my door which I beefed up on the bottom side of the floor which cross angle pieces. I really didn't see any water damage per say but the floor was definitely spongy when stepping on it going through my access door. Later I put in a couple of right angle sections secured to both floor cross braces and it stopped the floor from being spongy just inside the door. My floor uses the OSB rather thick flooring which is painted black on the bottom...
Think I am ok with all of this... No extra seal needed here between the roof and the top of the half door... This same type of seal is also used on the bottom of the top half of the door when it is folded down from the ceiling when being setup. Good tight seal between the upper and lower doors here...
Thanks for the tip however...
Roy Ken