charging of 2 camper batteries with truck alternator?

Knowing the voltage is nice, but I think the real question is how many AMPS are flowing back to the house batteries?

I'm sort of under the impression that the engine computer monitors the state of charge of the starting battery, and cuts back the alternator output to prevent over charging IT.

It would also be nice, for the purposes of this discussion, if we had some idea how many amp hours we needed to replace in the house batteries.