Van with Solar power but no auxillary battery and only on sunny days. Do I need switches and isolators?
I would separate the solar panel with charge controller and its own batteries. That way the vehicle can be left as "standard" - handy if you need an ordinary garage to fix it...

There are charge controllers available where you can also charge the leisure batteries when the engine is running, and with some you can control that facility.

And, as for fuses and isolators, YES you need them - controlling is much better than fire...