I'm soon to begib to repair some water ingress to out caravan, at the
front, which is a large fibreglass panel. With three wondow openings.
The window openings are framed with timber, which I assume is glued to
the inner face of the fibreglass.

I am expecting to have to replace much of the timber, what would be the
best adhesive to glue the timber frame to the fibreglass please?