If it's on the side and not the rubber roof then use Geocel ProflexRV caulk. Dicor is used pretty much for the roof only or horizontal surfaces.
You can use 3M caulk remover to help get the old caulk off it's being stubborn, but I have found that all you really need is a plastic putty knife. I then use goof off to wipe down and clean the area before applying the new caulk.
If your slide is leaking due to the gasket then you may need a new slide seal. Hard to tell without seeing it.
2006 Keystone Outback Sydney 30 FRKS
1997 Ford F350 Auto, 4.10LS Axle, 160,000 Miles, Crew Cab with DRW.
US Army 1984-2016.