Your FE has an E-350 chassis which has a GVWR of 12,500 lbs and a GCWR of 18,500 lbs (found here:
https://www.freedom-motorhomes.com/freedom-elite/specs/ ).
Take your RV to a scale (like a
Cat Scale) and weigh it. Preferably full (e.g. full gas, full water, full propane, full of your stuff, etc.).
How much you can tow is the GCWR minus the scaled weight or your hitch rating--whichever is lower.
Since you're planning on using a dolly you don't really have to worry about the tongue weight as dolly's don't come close to the 500 lb max.