bull, earlier 5th wheels only had a piece of side trim that didn't entirely cover the edge of the wood floor. It appears they thought the black coating on the edge would prevent the problem. Another issue that causes water intrusion is the placement of the floor support rollers. If they are too close to the edge of the trailer, they catch on the bottom slide trim flange and cause the trim to flex. This opens up the outside upper edge of the flange allowing water to get in.
I need to take a couple pictures but a few days ago, I tried adding a 12" square steel plate to the outside corners of the slide floor. This will hoefully serve two purposes. One , to support the slide's weight while sitting on the rollers during travel. Two, to prevent the roller from forcing that trim flange from opening. So far it looks good but I havent traveled yet to test the strength of the steel. I used 16 gauge sheets. (not quite 1/16th thick).
To answer your question, yes, if your floor is bad on those corners, it has to be removed to replace the floor. My hope is that my fix will prevent further water intrusion and support the slide. I'll know when we make the trip to Goshen next month.