Personally, I would want to stay below 3,500lbs because I am conservative when it comes to tow weights, but with your E450, it should handle it OK.
The GVWR for the E450 is 14,500lbs, while the CGWR is 22,000lbs, so that really gives you a minimum of 7,500lbs of theoretical towing capacity, give or take. But you may be limited by a 5,000lb hitch capacity as Thor just started putting 8,000lb hitches on their coaches this year.
At any rate, you should be able to tow your Saturn as long as you don't overload it with cargo. You can probably also tow it with say 1/4 tank of gas to save maybe 100lbs of towed weight.
The only thing that works on a RV is the owner...