I've been researching this currently as I am thinking about a F450 and a larger 5th wheel in the future. Had a Dually and 5th in the past but I was not over the 26,001 lbs GCW.
I am in Pa and like said before if over 26,001 lbs towing you do need a Class A. If diesel pusher etc over the 26k you need a Class B Non commercial license.
Now the kicker is if you have a F450 with a 14000lb GVWR and a 5th wheel that has a 14000lb GVWR the state see's that at 28000lbs no matter what even if you are pulling under the 26,001 because your trailer is not fully loaded. They will go by the possible GCWR of both vehicles.
$5 for the Class A non commercial permit and you must have another Class A non commercial or higher licensed person with you when you take the driving test and a truck trailer over that 26,001lb to test on.