It will help if you can tell us this
What permanent process was used on your main roof?
What type of roof membrane ( EPDM, TPO, PVC etc.) is on your slide roof?
I have coated slide roofs with the same material as the main roof. But in this case, both slide and main roof were the same material and the coating did not damage the slide seals.
A slide topper helps with keeping water out of the camper and sun damage away from top slide seal. There is merit to doing a roof coating on the slide plus a topper but it all depends on how you use the camper and how long you plan on keeping it.
Hope this helps
John