The annual stuff isn't bad - an oil change w/filter, fuel filters (2), front wheel hubs, and a chassis lube. All stuff you can do yourself. Your generatoir also should have an annual service.
But there are several other items that typically come every 24,36 or 48 months and some of them will fall due each year too. Air dryer change, coolant (add SCA or replace coolant), maybe a coolant filter, air filter, rear axle lubricant change, transmission fluid change (or a fluid analysis in lieu of a change), hydraulic fluid change (and maybe a filter). If you don't have the chassis service requirements for your chassis (see owners manual), you need to get them. If its a Freightliner chassis, they can be downloaded from the Freightliner Custom Chassis web site. Not sure about Spartan chassis.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL