First we need to establish a baseline.
Disconnect the 2 batteries on the trailer and fully charge them. Disconnect the charger and let them "rest" for at least 30 minutes.
What is the voltage on each battery ? If there is even a small difference in voltage on each battery, the lower voltage battery will drain the higher voltage one, even with no load. You can prove this with you ammeter !
With the engine running, what is the voltage at the truck battery terminals ? Now what is the voltage at the Anderson connector ?
Now plug the vehicle and RV together and measure the voltage at the RV batteries with no load from the RV. Last measure the voltage from the vehicle batter positive terminal to the positive terminal on the RV batteries. If life was perfect, the voltage drop would be zero.
REMEMBER, CURRENT FLOWS FROM HIGH VOLTAGE TO LOW VOLTAGE.
What you really want is about 13.8v-14.0V AT THE RV BATTERY TERMINALS, but you have no way to control this ! The vehicle voltage regulator is "sensing" voltage far from those batteries. If you had an alternator with an externally adjustable regulator, you could crank it up to the desired voltage, but the battery in the vehicle would probably be seeing close to 15.0V and it will be boiled dry.
You will never be able to "balance" two power sources that are separated by a cable that has more than ZERO ohms resistance and voltage is being sensed at only one location. These two power source need to be SEPARATED and some electronics employed at each end to properly regulate the voltage. They do make DC-DC converters that can do this, but a high current one would be VERY expensive.
Something worth trying. Wire an Anderson connector off of the feed to the inverter. When you want to use the truck as the primary power source, connect it up directly to that connector, bypassing the RV batteries. Assuming the inverter is also powering your RV converter/charger AND the converter/charger is still wired to the RV batteries, they will be properly recharged.
