If the seals are dry, crusty and shrinking so as to be able to see cracks, it is time to remove the window, scrape and clean off all the old sealant, and apply butyl tape around the window. Do not apply silicone over old sealant and I would not use silicone as a seal for my windows. You can buy butyl tape at RV supply stores or try one of the big box retailers.
When putting the window back in, just tighten the screws enough so that the butyl tape starts to squeeze out and then tighten a tiny bit more. Carefully trim off the excess with a plastic knife or something similar.
2008 Cougar 5th Wheel 27RKS
2005 2500 GMC Duramax