So what is the solution? I don’t know
OK, I'll try again.
The problems to solve, in order of priority, are
1) isolation between house and engine battery
2) avoiding alternator overload
3) line voltage drop to a trailer house battery
4) full and effective charging of house battery
5) rapid charging of house battery
Isolation is the most standard RV install. Overload is usually handled by fusing. And that is as far as a 'normal' RV install usually goes. Usually works OK for most folks.
To take care of effective charging, the Balmer Link looks like a good way to go but line drop might be a problem. I can't tell if it can be set to limit charge current but it appears to have the intelligence needed to do so.
Another idea, such as I suggested earlier, is at http://www.powerstream.com/DCC.htm
This a 4 stage, 3 level, 8 amp 12v
battery charger that uses an input voltage of 10 to 15 volts. Overall charge efficiency is about 80%. You put this in your trailer near the battery and you should get a decent charge in transit without having to worry about overloading the truck alternator or line volt drops or improper truck supply causing improper charging.
Or you could just put a decent inverter in the Truck, run 110v back to the trailer, and use a decent converter and charger (that you should have in the trailer, anyway) to charge the battery.