You guys are scaring me alittle bit hehe... I am planning to use this SMART CHARGING SIACR Relay made by Blue Sea Mdl 7650.
The Truck Start battery will tell the alternator when it needs to be charged in its normal way. The truck alternator will then go to 14.4VDC boost charge mode as required. The SIACR smart relay will sense when the start battery is being charged at 14.4VDC mode and will switch the Trailer batteries in parallel with the truck start battery and be charged at the same time. This also has a manual switch in for you to parallel the batteries as well. This is what I was going to play with this coming new season when I add my two battery banks to my truck and Off-Road POPUP trailer.
I'm guessing this is going to be a non-directional system. It will all hinge on when the truck start battery is being charged.

Roy Ken