It looks as if you will have coach batteries charging while running due to the ignition sense wire. However, without the coach battery wire your device will not sense the coach batteries being charged by shoreline or generator and will not charge the engine batteries.
The large solenoid below the device is the combining relay. Be aware they can get hot to the touch. You can add a jumper right to the large terminal associated with the coach batteries.
Before doing this you should determine if your solenoid is working correctly. Even if it is clunking when powered the internal contacts are a know failure item. If you do a search for Bird etc you will find pages of stuff. I have a dozen diagrams and notes on the various setups. To determine if it is working correctly check your engine battery voltage and coach battery voltage with the engine off and no shoreline. Now start the rv and do the same measurement. If the voltages are exactly the same you are good to go. If you see 1/2 of a volt difference across the two heavy terminals then the solenoid is sick.
Or connect shoreline, measure the batteries again. If you see a difference the solenoid is probably not activated and you can activate it with a light jumper from one of the small terminals to either of the large ones. You should be able to identify which small terminal is ground and which is the input control. If you short from the ground to the large posts thing will get exciting fast. Don't do it. When you do activate the solenoid with the jumper it might startle you with a large clunk sounds. If your fingers are on the bare end of the jumper wire you may get a harmless jolt. No worries, it will only make you jump.
Myron & Deborah
08 HR Endeavor 40 SKQ
2012 Ford Edge Limited FWD 3.5L
Aluminator tow bar, Demco base plate