Misfire, poor fuel mileage, 66,000mi no check engine lights on. New fuel filter.

"Gas treatments" are of little help most of the time with vehicles of the modern day and fuel that's available. With the misfire, I'd definitely check for diagnostic trouble codes. If there are none, a cylinder balance test and relative compression test with an appropriate diagnostic tool should be carried out. This could identity a cylinder or cylinders of concern to focus on. A base engine compression test isn't a bad idea to ensure no base engine issues are present also. Lastly, for your stalling, check your PCV hose for a possible leak. This would usually set a "lean" code for both cylinders (P0171 / P0174), but may not. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops