Battery isolators are often just big diodes that block the engine battery current from flowing back into the RV battery side of the circuit.
This keeps power in the engine battery to start and run the engine if the RV side runs the battery down overnight.
When the engine starts, both batteries charge but they cannot discharge back and forth between each side of the circuit.
The diodes do not improve charging or increase power in any way. They actually rob 1/2 volt from you from the voltage drop across the diode.