[kaptain]

@kaptain / Joe - That's a great schematic/photos.

I've probably read this too quickly or misunderstood something; however, what would be the reason not to simply go with a VSR (Voltage Sensitive Relay/Solenoid).

In other words, the alternator is direct-wired to the chassis-batteries; no Isolator/extra diode inbetween.  Then, upon voltage reaching a suitable level, the solenoid connects the house batteries.

The level is typically one that must exceed a 12v battery output (eg 13.8v), indicating the alternator is effectively outputting current without substantial drain.  As long as the alternator is running, the higher voltage automatically directs it to where it's most needed; you don't have an issue where an isolator is required in that situation because the alternator is always the highest-voltage source.

If you had different chemistries (eg LiFePO4 for the house batteries), this wouldn't be ideal; you'd be better off isolating with a DC-to-DC charger to gain some additional intelligence/logic; however, with SLA/AGM on both sides, I don't understand the benefit to a traditional isolator over the easier/less-expensive voltage-sensing switch.

I suspect I'm missing something obvious, but I felt it couldn't hurt mentioning on the off chance it inspired any new approaches

Using a Voltage Sensitive Relay/Solenoid (VSR/VSS) might be a good option, provided the on/off triggering voltage is significantly different. Why? I see a potential problem using such electrical-driven mechanical here. The sudden click-on/click-off of the charging circuit can create a significant electric flow between 2 battery groups, it may not behave well on a traveling coach. 

Let's say the trigger voltage is 13.5 to turn on the circuitry to charge house-Batt. We have the engine on, the alternator working, and the coach is traveling. 
In the beginning, the voltage of the Chassis-Batt could be around 12v right after cranking the engine; at the moment, the Alternator only charges the Chassis-Batt, because trigger voltage has not yet reached.
Let's say the voltage has just reached 13.50v, the triggering voltage. VSR clicks on, and the House-Batt voltage immediately starts to rise from 12.0v
The sudden switch-on of the VSR (effectively merge) can be a big load on the alternator and the Chassis-B, which, therefore, causes the voltage of the chassis-Batt drops quickly (it wants to get equal to the voltage of the House), now the alternator also has a higher load because more batteries to bring up voltage. Let's say Chassis-Batt quickly dropped below $13.50, thus causing VSR cut off the changing House-Batt. Chassis-Batt voltage rises again above 13.5v, and the cycle repeats. Now the VSR is acting like a flip-flop circuitry while charging both battery groups until both voltages of both battery groups have converged and stabilized above 13.5v, now the VSR stays closed, allowing both batt groups to charge.

Then you might have other devices that affect the voltage of the traveling coach, let’s say you just started to cook dinner, inverter comes on, drawing electricity from both battery groups until voltage goes below tigger 13.5, VSR opens….
I see it won’t be stable using a VSR, the original design concept ALWAYS separating 2 battery groups should be kept. We still have the option to use the merge switch.