The 3-pin connector on the Schaudt CSV409A which you refer to are designed to be used in conjunction with the Schaudt LRM1218 solar regulator which like the CSV409A can handle 2 batteries (starter + leisure).
In the Eriba vans, pin 2 is unused as there is no starter battery connected to the CSV409A, pin 1 is routed through the circuit board to the battery negative terminal, pin 3 is routed via an onboard 15A fuse to the leisure battery positive terminal.
You can therefore connect using a suitable cable & plug via the CSV409A and make use of the internal 15A fuse, or you can connect your solar regulator directly to the battery, not forgetting to add your own inline fuse of course.
Hope that helps!