Your GCWR is only 13,000 pounds, which does not leave a whole lot of towing capacity.
GCWR-(actual weight) = tow capacity. Keep in mind the same chassis with the V10 is rated quite a bit higher,and GCWR is mostly a performance rating. In other words... Going over 13,000 pounds will slow you down and burn more fuel, but performance is subjective.
In addition, you have to be careful that the tongue weight will not overload your rear axle. If you have not weighed it yet, you'll want to, to ensure you even have rear axle capacity left for a trailer. Running too heavy in the rear usually means too light on the front end, and that can cause handling problems. A weight distribution hitch can help mitigate that, but you will need to upgrade from the standard RV hitch to use weight distribution (E350 van receivers fit). You may need to do that anyways because most RVs of that era only had a 3,500# hitch receiver.
