Once you have a fouled fuel system it can be multiple items that need attention. Start simple and work up. Start with carb and throttle body cleaner and drain the carb. If you still have rough idle then you can just remove the carb and have a shop dump it in a parts cleaner and then blow it out. Using a couple sprays of starting fluid at the air inlet to help get the first start. If that fails you can go for rebuild. A flush of the fuel line would not hurt either as you may have crud in their as well.
If you have an in-tank fuel pump there is usually a "sock" filter on the fuel pump and if that gets plugged it could result in poor idle. On my Four Winns the fuel pump sits on the front of the motor (5.7L) and then has the in-line filter. I don't know what is down in the tank itself but you can call the dealership and they can probably tell you if it's just a suction line or more than some kind of basic screen.
Unless you actively turned the idle screw it's unlikely it turned on its own. Don't mess with it is my recommendation.
Bad plugs typically result in poor/hard starting, not bad idle. Bad coil/distributor typically results in missing/backfire and have a "you know it when you hear it" sound. So far you've not described either.
I use the Sta-Bil product as it has fuel system cleaner as well as corrosion inhibitors. I'd recommend pouring a bottle in your gas tank (certainly won't hurt).
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