Some comments:
-- It is probable that Overland are using a Blue Sea ACR or similar voltage sensing relay. These are a great solution assuming that your camper batteries and starter batteries are both grossly of the same type, eg., both lead acid.
-- A voltage sensing relay does NOT charge the starter battery first, it merely measures the voltage at one or both batteries and connects when either hits the target voltage (typically a bit over 13v) and disconnects when either drops to about 12.7v, the so-called "full" charge voltage of a lead acid battery. But, as the starter battery typically recovers almost immediately, and it is the camper battery that is deeply discharged, the net effect is close to being the same.
-- Assuming that your alternator charges at a bit over 14v AND that the cables between your starter battery and your camper battery are large enough, typically around 0 AWG, you will get very good performance.
How fast will your lead acid battery charge? A LOT slower than you think. Charge rate is determined by:
-- The difference between the voltages of the battery and the charge source. So the starting charge rate is the highest, dropping as the voltage of the battery rises.
-- The depth of discharge. With lead acid, the highest rate is typically achieved when the battery is about 50%. But as noted, this rate drops rapidly as the battery voltage rises. According to Lifeline, you want a charger/alternator/generator which can supply more amps than the battery will take when it is taking at its highest rate. Anything more than that will NOT lead to faster charging. May keep the alternators from overheating, but you will not charge any faster than the battery will accept.
-- Finally, to avoid sulfation, the battery needs to be held at a high voltage (with low amps) for a L O N G time, 30 minutes to an hour for every 100Ah of battery.
I find it useful to think of things this way:
-- Alternator or generator - use for bulk charge.
-- Solar - use for absorb charge.
-- Shore power, use for both, assuming that your shore charger has a proper absorb stage. Some, eg. Progressive Dynamics, famously don't.
Lithium is a bit different:
-- Lithium batteries tend to have a much flatter curve - discharging the voltage tends to hold very stable until it drops and, when charging, it stays low until the end. For our discussion, that means that a lithium battery sucks up amps - which may be a danger to your alternator.
-- The resting or float voltage of a lithium bank tends to be around 13.5v, thus it is higher than the combine voltage of most intelligent relays. (Victron makes one with voltage settings for their lithium batteries.) So the relay will never open, potentially letting your starter battery, even fully charged, put a load on the camper battery.
-- The opening voltage of 12.7v is too low for most lithium batteries.
-- Scholars debate, but while most lithium banks are made of 3.65v batteries and this would tend to indicate a charge voltage of around 14.6v, there is a lot of anecdotal evidence, that lithium banks last longer when only charged to around 90%; call it 14v.
In the real world, where most of us can't get inside the vehicle's computer controlled wiring, a battery to battery charger is a better option for a lithium camper battery.
YMMV, caveat emptor, etc.
Life before RV was so carefree, I just plugged stuff into wall outlets 
