Onan uses several different park arrestor types, so the genset model number would be helpful to give an accurate answer. However, I don't know of any that need a special tool - just a suitable size and type wrench. Unless it has been removed regularly, it is probably semi-welded in place with the effect of high heat and corrosion. People sometimes end up using Vise Grips after the corners get ground off the bolt head. Plenty of penetrating oil (PB Blaster or similar), applied a day or two in advance, is a good idea too.
Mine happens to be a hex (6-sided) bolt, so a 6 point socket is best if it's corroded. Other models may have a 4-sided bolt, which needs a different wrench.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL