question

@missy the basic bottom line is that one battery bank cannot drain another without an application of specific current or load - ie, the device would not take current from your leisure batteries to give to the starter battery unless there was a load on the starter battery that was draining it, or (conceivably) unless the starter battery was in such poor condition that it wasn't holding a charge at all, in which case it could conceivably present as a load that would be a constant drain on the system.

In regard to whether you have to install a switch; no, that's not technically required, and yes, you would simply install the green wire bridge connector into its position if that is what you choose to do.

That being said, I'm a big believer in maintaining the ability to isolate systems wherever possible when/if necessary, so I'd recommend two things: first, I'd personally recommend going ahead and wiring the switch terminal to an accessory-switched circuit in your Sprinter; thus, when your key is in the "on" position, the Orion will be on, and when the key is "off" so will be the Orion.

Second, I'd also recommend installing a 40A breaker (such as the Blue Sea/Bussman OEM Series 187F40) on each side of the Orion. This provides not only protection for your cabling but also the ability to isolate each side of your battery banks. This can be extremely useful in maintenance, such as if you need to adjust the Orion or access its connections and don't want to be dealing with live 12vDC while doing so; furthermore it adds the ability to completely shut off the device in the event that you're going to be parked for a while and don't want its minuscule self-consumption to be a factor in the life of your batteries.