If the thing is on long enough both battery banks will eventually equalize and you will wonder why things "are now fully charged".
Think of a slow draining sink. The P trap has become clogged but not completely. Cleaning it allows the flow of current back to normal.
Or, think of a clogged artery in a heart. Plenty of pressure on one side of the clog and little flow on the other side. Bypassing it with a set of jumper cables anywhere along the line such as right at the clog or further back such as right at the battery terminals results in bypassing the clog.
When you have solar charging both banks of batteries directly though a solar controller you are paralleling the Big Boy basically.
To test it further, turn on your headlights, dash fans, and anything else you can find that is running off the engine batteries. You should see a bigger difference in voltage across the terminals if the load is big enough.
Myron & Deborah
08 HR Endeavor 40 SKQ
2016 Ford F150 Limited V6 with Ready Brake
Aluminator tow bar, Demco base plate