There are some things I try to do once a year and spring may be just as good as other times -- I'm ignoring the anti-freeze, propane, water areas.
Check for cracks in sealants on the roof/sides.
Check tires for air pressure, cracks, lug nuts, etc. Tires more than 4 years old?
(
link on how to check tires)
As applicable, lube bearings in wheels/brakes/whatever.
Wash/wax the trailer.
Test all systems with enough time to fix anything before the first trip.