House Batteries not charging

[2015GL-Owner]

I installed two new AGM house batteries in my 2015 Interstate Grand Lounge. The batteries were charging good from engine power while driving, until I got a new solar panel controller installed!. My batteries were not charging with Solar panel and we got a new controller installed at the factory repair center in OH. Since then, the batteries have stopped charging from engine power while driving. I live 600 miles away so it is not possible for me to take it back to the repair center.

Our motorhome is parked outside in our driveway where it is mostly sunny. Looks like solar panels are constantly charging the house batteries. I got a voltage meter connected to the house batteries and it shows 13.1V and 100% charged. When I turn on the engine and run it at 2500 rpms, nothing changes in the batteries voltage. It stays at 13.1V. When we are driving and we turn ON the inverter to watch TV and keep the refrigerator running. we run out of batteries. I am suspecting that the solar panel continues charging the batteries and it is not switching to the engine power charging, even when the voltage drops. My solar panel only provides around 3-4 Amps so it is trickle charge for the batteries. I have following questions:

Q1 - At what voltage drop should the solar panel charging stops and switches to the charging form Engine power? how does this switching happens? Does the controller monitor Battery voltage and when it drops below certain level, it switches to alternate method of charging?

Q2 - Is there any way to turn off the solar panel charging so batteries are forced to charge from engine power?

Q3 - How do I access Solar panel controller? Where is it located? What does it look like?

Q4 - the house batteries can charge with Solar Panel, Engine Power, Generator, and shore power. How is it controlled? Who is responsible for switching the source and how is it decided?

We are planning a long cross country trip this summer and we want to make sure the house batteries are constantly charging so we can run TV and other appliance inside the van.