When I first installed my system I connected my engine battery to my house battery by a continuous duty solenoid because it was cheap and could tolerate the high amperage. This method worked fine, but required me to keep an eye on voltage and connected it when needed by a switch. The switch was connected to the D+ line so even in the on position the solenoid was only connected when the engine was running. However there is a voltage drop and the solenoid is always hot and it prevents the solar from charging the engine battery when off.
After doing research I decided to go with a Victron Intelligent Battery Combiner in the 230amp version (they make a 150amp version also). I picked this one because there is no/minimal voltage drop and it works both ways. If the engine is charging the battery then it connects the two batteries, but more importantly if the van is parked and the solar is charging the rear house battery it will connect them and charge both batteries, eliminating a need for a maintainer on the engine battery. This one only works if you have a common chassis ground between the two batteries (which I do).
Install was easy and seems to be doing well so far. Connects the two batteries with charge into either battery (13+V). It has an aux start mode, which is important to me also. Only concern is there is no off switch, which is why I have a circuit breaker and on/off that can disconnect the connection if I don't want the engine to charge the rear battery.
I also removed the 100amp fuse I was using and replaced it with a 150amp circuit breaker which allows me to disconnect it on the fly. My engine battery cable connects to my rear system through an 80amp fuse and line originally used for the rear AC which has been removed. So far that 80amp fuse has not blown so I imagine the draw is less than that.
I still use the same solar charge controller. It's been about 3 years and no issues with it, even though it is a very cheap model (UPDATE: Solar charge controller failed after 5 years).
