Adjust trailer brakes (about every 3,000 mi)
Clean, inspect and repack wheel bearings (refer to your manual for interval)
Check torque on suspension parts (u-bolts, spring hangers, etc.)
Grease wet bolts (if installed) every time you adjust the brakes
Inspect roof caulking & reseal as needed (at least twice per year)
Apply anti UV agent of your choice to all of the plastic (plumbing vents, a/c covers, skylight, roof vents, plastic door catches, etc.)
Flush water heater at least once per year and inspect/replace anode rod.
Wax/polish exterior every 6 months
Sanitize fresh water system as needed
There will be many different opinions on this subject from the other members
Added: I always used a dry lubricant that contains teflon on the slide gears.