Lead-acid batteries require two things to get fully charged:
DC-DC chargers address the first requirement, but unless you are driving for hours the isolator still won't have enough time to fully charge the batteries. DC-DC chargers were first marketed for cruisers (powered boats) since they run the engines (“cruise”) for hours each day.
Charger throughput is generally limited to 20-30A since higher rates would dictate $$$ internal components. This isn't as big a limitation as it might seem, since lead-acid current acceptance drops substantially throughout Absorption. Untested theory: b2b chargers might work well beside normal isolators; this is essentially the $$$ CTEK SmartPass setup described below.
DC-DC chargers come in different ratings (10A, 40A, 60A, etc) which increase significantly in price as the rating increases.
Bigger is not always better, since battery types accept current at different rates and buying more charging capacity than the battery wants3) is a waste. Assuming a 100Ah battery:
The most common b2b charger with 'dwellers is the D250 series, all of which are 20A smart chargers with integrated MPPT:
House and starter batteries are combined when the starter battery holds >13.1v for 5 seconds.4) The batteries are isolated when the starter battery is “<12.8V, for 10 sec… or service battery voltage > starter battery voltage”.
The SmartPass accessory adds 80A-120A (see below) of alternator-voltage charging to the D250's output when Vbatt is low enough that the alternator can still charge it directly. This combined charging5) will happen in the first half of Bulk mode. As house battery voltage rises to normal alternator voltage the smartpass can no longer contribute. At that point the 20A D250SA continues voltage-boosted charging to Absorption voltage.6)
The MPPT charger will charge the house battery at up to 20A. CTEK specifies 50-300W of panel, though the 23v Vmax limits the unit to [electrical:solar:panels#panel_voltage|nominal 12v panels]. Note: polycrystalline panels may be more useful here, as their Voc should be a bit lower than mono.
The D250-series trickle charges the starter battery from a solar panel at intervals of 3 seconds if the service battery is fully charged.11)
The Sterling Pro Batt Ultra series is alternator-charging only, but can be configured to exact setpoints. The BB1260-12 is a 30A example and costs about $50 more than the D250S above.
Redarc makes a 25A b2b charge with MPPT similar to the CTEK.
Kisae makes a DMT1230 DC-DC charger 30A charger with 45v MPPT input. They also make a 50A model (DMT1250).
Customer support is generally regarded as excellent.
Victron makes several DC-DC chargers, some of which are configurable by bluetooth. 12v examples: