I've left them fully extended for 6 months or more at a time in the wet, rainy Pacific Northwest with NO problems even after 13 years. Never even cleaned the rams off at all. We've always had HWH systems though, others may be more problem.
2009 45' Magna 630 w/Cummins ISX 650 HP/1950 Lbs Ft
Charter Good Sam Lifetime Member, FMCA, SKP
RV'ing since 1957, NRA Benefactor Life, towing '05 Odyssey