Based on a few decades of fumbling with sealers, silicone caulk hardens.
Along about the time of the first bump, the body flexes, the windows/vents/fans shift, and the bond breaks... necessitating a second/third/fifteenth layer of silicone caulk.
Which immediately breaks.
2003, for our ExpeditionVehicle, I seated my windows using Vulcum 116.
This's available in a few colors -- tan, white, gray -- so matching it to our 'almond' window-frames and tan rattle-can paint-job was a cinch.
Nearly two decades of bouncing up rough logger tracks and across rivers/deserts/California roads, and the stuff is still pliable.
For our ExpeditionVehicle, we built using a rigid commercial box engineered for a million miles of worse than we could throw at it.
Of course, we have zero-zero-zero holes in the roof [like every RecreationVehicle [spit]], so your experience may differ.