When you realize that the roof thickness in the slide-out is much less than the main roof (so limited insulation), anything that reduces heat gain will help. How much? I don't know, but if you have an IR temp gun, then when your trailer is parked half in shade and half in direct sun light, compare roof temps in both areas.
Larry Day, Texas Baptist Men volunteer
'13 Silverado LT 3500HD D/A CCSB 2wd, custom RKI bed
'19 Starcraft Telluride 292RLS