JEgan,
I'll clarify a couple of issues;
- SAE rates class IV hitches at 500 lbs tongue weight and 5,000 lbs towing capacity without weight distributing bars and 1,000 lbs tongue weight and 10,000 lbs towing with weight distributing bars.
- We have contradictory information on the factory hitch saying it is good for 12,000 lbs (towing with wd bars) and also that it is a class IV, but nonetheless you should be safe within the manufacturers rating.
- The factory 3rd gen hitch appears to be a more robust design than earlier models.
- Without knowing the tongue weight and GVWR of your trailer (which you don't have, yet), I technically can't answer your question. That having been said, I would be fairly comfortable suggesting you will be withing the ratings of the stock hitch as long as you use the weight distributing bars. You'll need to weigh any potential trailer to know for sure.
I think everyone else covered the 5er vs. trailer issue well enough
hope this helps
Dave