Hi Chuck
The Toyota regulator is not a smart regulator (as in staged charger)
Sterling have developed a Special Battery to battery charger to overcome the fact that some manufacturers are incorporating the regulator into the engine management systems, This takes power from the starter battery and converts it to staged charging for the leisure battery('s) Obviously the starter battery does not benefit from the staged charging
http://www.sterling-power.com/htm/btb.htm
These are more expensive than a standard smart regulator, but have the advantage of being easier to fit and hence easier to swap into your next vehicle.
Starter and leisure batteries are supposed to have different constructions, in reality there is little if any difference, My local camper shop had their van battery fail, so they stuck a leisure battery in to get them going, several years later it was still going strong.
Talked to them about a battery isolator and it will work but it will still mainly charge the starting battery. The second battery will charge but will take longer to charge do to the regulator think it is a load.
Classic example of vehicle electricians talking rot, it is "semi true" but as nothing to do with the regulator "thinking"
Consider
Starter battery only ever expend's a fraction of an AmpHour starting the vehicle and then is constantly fed while the engine is running, ie if the system stays at 65 % charged it will hardly move from 65 % charged even while starting.
A leisure battery however is often much lower than 65 % and much higher (if its charged via a staged charger it can be 100% charged)
This is reason one why it takes longer to charge (if its at one of the Lower points)
Secondly the leisure battery is subject to voltage drop, from the Diode isolator and the wire (second batteries are usually much further away)
Hi Paul
I am impressed with your auto electricians, you will be amazed to know that they are the first and only auto-electricians in the world (so far) who have not got that wrong...
Hi Leon
Reduces by a fraction but there is still voltage drop and enough to ruin charging...
I understand your sense line point and if this is done a much better charge can be achieved with a diode isolater.
Note of caution, The reason they moved the sensing wire to internal on the alternator (or short external run to the output 1 or 2 inches away) is that if this wire breaks, the alternator goes into balls out mode and destroys itself (and occasionally some of the vehicle electrics !) so do be extremelycareful about anything you do with this wire.
I have to disagree about the "most practical can be achieved" practical is a relative word adding a smart regulator or B2Battery charger is easily achieved, with much better results.