I recently wanted to know the size of my skylight. I contacted Jayco at this link:Jayco Customer Service
. I filled in the form and told them what I wanted and they responded quickly.
I've not installed my new skylight yet, but I think all it involves is scraping away most of the old lap sealant and removing the screws. I'm sure the old skylight will be stuck down really well, and that removing it will take a while. Great caution will be used so that the roof membrane is not damaged.
When you replace your old skylight, consider getting a new one made of Lexan; much more resistant to damage (hail, tree limbs, etc) than most plastics. One place that sells RV skylights and has some instructions about installing them is here
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch