Alternator + solar charging has considerable benefits over alternator charging alone, and over solar alone. It can make a small solar install punch well above its weight, acting like a much bigger install in areas of poor insolation.
Solar works anytime there is available sunlight, not just when the vehicle is running.
Alt charging alone is unlikely to get lead-acid chemistries fully charged since Absorption can take hours and alternators often cannot hold the required voltage. Incomplete charge (Partial State of Charge, or PSOC) over time will result in battery murder.
How to avoid (or attenuate) battery murder with this scenario:
The reason for this is related to how lead-acid banks charge; they can take a ton of current in the first ~80% of charging (Bulk) then they hit a kind of wall. At that point you could drive 100mph or hook the bank up to the Hoover Dam's electrical output and it wouldn't finish charging significantly faster.
From ~80% to 100% (Absorption stage) charging requires little current but a long time holding at a relatively high voltage. So in our alt+solar scenario the alternator does the heavy lifting in the beginning then a small solar install does the light (but lengthy) work at the end.2)
Solar power is relatively weak compared to alternator charging. Solar is most challenged in the morning when the bank is deeply discharged. The sun is weak then but the bank needs a large supply of current (early Bulk charging).
PWM controllers are especially hamstrung in this scenario; since they run the panels at battery voltage (Vbatt) they produce the least power exactly when the most power is required. By the time Valt is reached PWM controllers are able to produce meaningful power.
Alt charging shovels current into the bank up to the point that alternator voltage (Valt) is reached. Solar can take it from there. A good metaphor might be a two-stage rocket: the alternator does the heavy lift getting the rocket off the ground then the solar charging takes care of the high altitude, high speed duties.
Boosting solar charging with alternator, genny, etc, works best during bulk stage. This is true for alternators because their terminal voltage is rather low, and for both generators and alternators because Absorption stage takes little current and a great deal of time.
In practice, it is generally ideal to start charging from 50% depth of discharge for best fuel efficiency. For most efficient use of fuel, generator charging would cease when Absorption voltage is attained and current has fallen off to a point solar can handle it. Alternator charging would cease when house battery voltage reaches alternator voltage; once solar takes the voltage higher than the alternator puts out the alternator is no longer contributing.
When the chassis and house batteries are connected, deep cycle charging voltages3) from solar or other sources will be passed to the chassis. Since charging voltages differ4) this could be an issue if house batteries were flooded; flooded starter batteries would require more frequent watering. “Maintenance-free” sealed flooded batteries might be expected to fail earlier due to water loss. AGM starter batteries might be overcharged by flooded charging voltages, though voltage drop across the relay and inter-battery wiring may serve to attenuate that possibility.
If overvoltage is a concern, one can use one of these workarounds:
Voltage sensing relays can get "stuck connected" if the alternator and solar were charging the same time. This situation would exist until house voltage dropped below the VSR's disconnection setpoint.6)
It is effectively the same issue as the issue above. Workarounds: