If you have the pebble style roof, which some popups have, don't use eternabond. I wrote them last year and they suggested that it should only be used on smooth surfaces. Won't bond correctly adhere due to the rough surface. That's from eternabond, not me.
I used GE silicone sealant for an emergency sealing last year in MT; all I could find. It worked, but that stuff is not UV friendly and I just peeled it off a week or two ago. It had totally hardened and started to peel upward at the edges, Remained water tight, but not for long.
I replaced with Geoflex RV on the fantastic vent cover and Dicor on all other sealant areas. But used white; not sure if they have clear or not.
As for Flexseal, consumers reports didn't give it a very good rating.
Trailer: Lifted 228BH, heavy duty springs and Yokohama tires
TV: 2016 GMC Sierra Z71 4x4 CC, SLT
Spare TV: Two Alaskan Malamutes
Living somewhere in ID; previously lived in Moab UT; previous to that, don't ask!