My understanding of solar approved cables is that they are suitable for permanent installation in all weather conditions without protection, halogen free, double insulated and now conform to a newer standard. This is where you will have to change those cables, the new standard. I believe that once the cables go into your van they would not have to be solar cable as they are no longer exposed to the elements. But that is my belief.
Does your existing van have a junction box on the roof that the solar cables go into and are joined with another cable that goes to your controller? Solar panels will now have to have an isolator on them before connecting to the controller.
All cables connected to the battery have to be a suitable size and fused accordingly. If you used busbar or studs for power connections cables suitably sized and fused. As your batteries are outside the liveable area and you say are vented to the outside that's fine. Batteries have to be securely fixed.
If your going to get an electrician to certify it make sure you find an electrician that actually fully knows and understands what goes with 12v wiring and regulations on caravans. Most electricians will not know these things. This area is not common practice with sparkles as most would never have worked on one.
Don't know if when you sell a van it has to be done with certification papers or not. When I sold our van, which was not registered, and was an on site caravan which had to be removed, I sold it "as is" and it was in the bill of sale.