Once a year. tear down and lube hubs, lube coupler and oil hinges, touch up paint with rattle can.
Every use walk around inspection, Lights coupler, Hardware, tires,
Wash time with the truck, hitch them up and scrub, tire dressing.
Wheel bearings, wheel bearings, and wheel bearings. Also check frame and such for cracks, loose bolts, etc. Checking the lights, lugnuts and tire pressure should be done every trip.