burlmart - Having been around engineers for the last 40 or more years, I know exactly what you are talking about. However, as you stated, ratings are for a reason - to provide that margin of safety. Much like roof loadings, floor loadings, bridge ratings etc. a margin is needed for those unexpected events. On an RV, it could be side loadings going around a curve, hitting a curb or pothole, or some other unexpected object. I agree that 100 lbs or maybe even a 1,000 lbs may not make the difference, but it does decrease that safety margin.
My real question for "Mainer1" was one of interest in the ride quality. My rig has so much excess capacity that the ride on the suspension could jar'rrrr false teeth loose. I have to deflate the tires for a cushioning effect, yet not exceed the capacity of the tires at a lower pressure.