If you save and reuse the perimeter aluminum frame pieces it isn't too difficult to rebuild a Ventura
The interior core (that rots) looks like it was an interior closet door slab. To remediate, I sourced a piece of paneling that matched the interior, a powder- coated skin for the exterior and a small fixed (non-opening) RV window. The original opening window allows water to penetrate.
Rebuilding a hollow wood core door, sized to the original dimensions, was straight forward from there.
Caulking the skin to the aluminum edge pieces protects the wood from water ingress, but also assemble the wood core with waterproof wood glue just in case.
The "new" door is as good as new after 5 years and about 50,000 kilometers of use in all weather conditions.