twinboat: Well, let me explain it like this:
The goal is to arrive at your boondock-camp spot with full 100% charge on those house batteries so you do not have to run your generator until your house batteries drop to ~11.9V or 40-50% SOC.
To accomplish this, you can very easily connect the Keyline VSR (or knockoff device) by running 1 positive cable from the VSR post to your house battery... and a 2nd positive cable from the VSR to the Chassis-Engine Battery. It is very easy. Just be sure the cables are long enough to extend your battery slideout trays.
So, I am saying I was able to easily mount the Keyline on the left side of my battery compartment (wall).
Problem solved.
For a technical explanation, I will refer you to the KeyLine website:
https://www.keylinechargers.com/prod...-artic-cat-etc
I would describe it like this: The Keyline is both a battery isolator and a battery combiner.
Blue Sea has the marine market and KeyLine penetrated the RV market, but let their patent go about 5 years ago, so now there are cheaper devices, probably made by the same Chinese company, but you can't know if they use the same quality components as KeyLine does. Anyway, over the years the Keyline VSR price dropped from about $150 to $90.
This VSR is not smart. It is just has internal relays that Cut-in and Cut-out at certain voltages. I.e., when one battery drops too low the circuit open and that keeps one battery back from draining the other. And it's not controlled by a key switch or your Battery Disconnect Solenoid (aka Salesman switch by the front door.)
So, this VSR works to charge both batteries whenever the alternator is running or when your solar charge controller is active.
I have found this device works flawlessly for over 7 years. I had one in my previous RV and now I have one my current RV. I wouldn't live without it!
And with so many posts, like this one, every time I read about battery short problems, 9 times out of 10 it is the BIRD or Trik-L; and the Amp-A is just a bigger brother to the Trik-L. So, this is why I say: "Yank these out and start using a VSR."
https://www.keylinechargers.com/prod...-artic-cat-etc
Amazon $94 (with battery cables):
https://www.amazon.com/Dual-Battery-.../dp/B00WTAFR5W
No need to install a resettable fuse either! Just direct wire it. (I used left over solar 10 gauge cable.) However, I have a diesel and everything is in the back. Also, my engine and house battery banks are stacked one on top of the other.
Remember, you just connect positive-to-positive. No negative cable is required since both the engine and house batteries shard a common chassis ground.
==== NEXT TIP ===
Stop using your antiquated charger in your Inverter/Charger and buy a Victron AC-DC 17A or 25A charger to be your primary charger -- and turn off your charger inside your coach. (It will still work for when you need overnight charging, but otherwise it's junk!) And if you are wondering why your house batteries are not lasting more than 2 seasons, IMO, I would bet it's your Inverter/Charger that is "cooking" your FLAs!
Once installed the Victron, I not only have bluetooth monitoring on my cell phone, I also only add about 1/2" of water every season; and now my FLA-House Batteries are lasting 5+ years!
Moreover, when you put your RV in storage, you don't want to leave your Salesman Switch "on" for your BIRD to work. And with the Victron you just run an outside extension cord to it and you are done! Which is another perfect solution to maintaining your house and your engine battery -- with the KeyLine VSR?Combiner.
Note: I mounted the Victron AC-DC in my bedroom and you just plug it in when you have shore power. And when you put your RV in storage, you just unplug and pass the Victron power cord outside. (I have a tip-out window so it's easy and quick.)
Here's the link:
https://www.amazon.com/Victron-Energ.../dp/B07TJK3X71