Our batteries are in parallel. I have measured current flowing to/from each battery for any particular load or charge and have determined that they are not balanced. In parallel, ideally the load should be supplied equally from each battery, it is not. The difference can be substantial, especially when starting. Same with charge current--one battery is always favored over the other. Voltage at the battery terminals generally remains the same, but current flow through each does not. I am from a school that seeks to balance batteries in any given situation where they are connected in a bank. I'm not interested in dealing with that on this truck.
For the shorter term, putting either battery on a Minder by connecting the (+) to the (+) of one battery and the (-) to the other battery or connecting the Minder to a single battery isn't going to make that much difference. Technically speaking, how the batteries are connected and a charger to the bank does make a difference in the longer term. It can be mitigated somewhat by interchanging the batteries' positions every so often, but I'm not even interested in doing that.
On my Powerstroke running dual service deep cycle/starting AGMs it's the same thing with the imbalance. I condition them IAW battery mfr procedure separately every so often when it looks like one is not holding the same charge as the other and most recently interchanged them a second time in 8 years. Usually, it's one battery dragging the other down after some time in that unbalanced situation. The conditioning process corrects that. I seek to keep them within 100mv of each other after a 24 hour resting (no charge and no load for 24 hrs). They will be starting their 9th year in service in January.
One other note is that these OEM flooded batteries are not intended for cycling use as in operating DC loads in a camper, etc. They are starting batteries only. After a small number of deep cycles, they will be toast.
In many if not most cases where it's simply a charge line running from the truck (e.g., 7pin) to the RV, the truck batteries and RV battery are connected in parallel without an isolator, but there is usually a relay with a factory towing package that energizes the charge circuit to the RV through the 7pin when the ignition is ON.
When the truck is OFF, if the RV pigtail is still connected to the 7pin, the truck batteries should be out of the circuit truck because the relay is not energized. This charge line is so long and AWG so small, that the max charge you will see through it is 3 to 6 amps depending on a few factors. This line is intended only for keeping fully charged batteries topped up while in transit to/from your destination. If the RV batteries are fully charged you will see lesser current if no lights, etc are on when under way. You will also often see a substantial voltage drop between the truck's battery terminals and the RV battery terminals, maybe 14+ at the truck battery and in the 13s at the RV battery. This is not an ideal charge situation and should never be used as the primary means for charging RV batteries, especially from a deep cycle discharge.
