Originally Posted by Jim Foster
I hope they are right too. My question to them would be "How is the charge to the TV battery being cut down?"
Having the chassis battery fully charged doesn't limit charging output to the chassis battery. There's no vehicle with a properly wired charge line that works that way.
Modern alternators are voltage regulated. The voltage output, aside from temperature compensation, will remain constant until the alternator output approaches the maximum current rating. A growing number of vehicles have the power train computer dial back the output voltage a little after the engine has been running continuously for a certain period of time to prevent overcharging, but that's a small adjustment and is based on time and ambient temperature and doesn't make much difference.
As such the regulated voltage will be the same whether the chassis battery is fully charged or not. Less the resistive losses in the charge line and ground path, this is the voltage that will appear at the trailer.