If it was a boat we would use various methods to bond timber battening to a GRP hull.
These include glassing in with chopped strand mat and resin , Davids p40 for small areas,or sikaflex (screwfix). If an area is subject to particularly high loadings we might use epoxy resin with some microfibres (Google Robbins timber or try sp resins also Aquafax now supply epoxy).
My feeling is that for a motorhome sikaflex is probably the best option, but remember that battening out is like the foundations for a house, just because it is hidden doesn't diminish its importance, it should be neat and accurate and when the lining is in and the fit out complete it almost shouldn't matter if the battening comes detached ( with emphasis on almost) as the entire work should lock together to create an interlocking whole.
Good luck
Mart