I have a 4WD 2008 Sierra extended cab with the 5.3 liter and a 3.73 axle and it is rated for 7500 lbs. 2WD will add another 200 lbs in towing capacity. I pull a trailer that scales at 7200 lbs ready to camp with 900 lbs on the hitch. There is just me and my wife in the truck and I am careful about what goes into the truck bed since I am using most of the 1500 lb load capacity and all of the towing capacity. I tow locked at third gear at 60-62 mph and in 100 Deg. F. ambient temperatures, the transmission runs at 200 Deg. F. Grades of more than 2 or 3 percent will force a shift to second gear and if it is a long grade, I will slow down to 50 mph to drop the rpm down some. I also change out the transmission and rear differential fluids every 35k miles.
I would reiterate that you definitely want to scale the trailer and while I have no immediate plans to move to a different vehicle, in your case I would recommend a 2500 for the load in question.
A standard simply creates metrics for a named set of conditions. Just because the standard does not cover all possible conditions does not make it useless. That's like saying since I only have a yard stick, I can't possibly come to any conclusion about the dimensions of a sheet of plywood. Measurements derived according to a standard beats opinion any day of the week.
As others have indicated, this type of floor system has had issues. Manufacturers have made improvements in different ways. In 2010, Forest River increased the aluminum tubing embeded in the floor and went to 16 inch spacing. In 2009, Trial Lite (before they went bankrupt) added an extra luan layer on the top surface.
From my reading on the subject, it seems that the floor fails most often in the major traffic areas and if the top and/or bottom layer of luan separates from the foam, structural rigidity is lost and the floor will sag with weight. The foam rarely compresses. You may want to get a look at the bottom of the floor and have someone step on the "soft spot" to see if this is the case in your situtation. If so, others have fixed the problem by adding supports between the frame members under the soft spot.